xref: /macosx-10.9.5/CPANInternal-140/DateTime/lib/DateTime.pm

package DateTime;

use 5.006;

use strict;
use warnings;

use Carp;
use DateTime::Helpers;

our $VERSION;

BEGIN
{
    $VERSION = '0.53';

    my $loaded = 0;
    unless ( $ENV{PERL_DATETIME_PP} )
    {
        local $@;
	eval
	{
            require XSLoader;
            XSLoader::load( 'DateTime', $DateTime::VERSION );

            $DateTime::IsPurePerl = 0;
	};

	die $@ if $@ && $@ !~ /object version|loadable object/;

        $loaded = 1 unless $@;
    }

    if ($loaded)
    {
        require DateTimePPExtra
            unless defined &DateTime::_normalize_tai_seconds;
    }
    else
    {
        require DateTimePP;
    }
}

use DateTime::Duration;
use DateTime::Locale 0.40;
use DateTime::TimeZone 0.59;
use Time::Local qw( timegm_nocheck );
use Params::Validate qw( validate validate_pos SCALAR BOOLEAN HASHREF OBJECT );

# for some reason, overloading doesn't work unless fallback is listed
# early.
#
# 3rd parameter ( $_[2] ) means the parameters are 'reversed'.
# see: "Calling conventions for binary operations" in overload docs.
#
use overload ( 'fallback' => 1,
               '<=>' => '_compare_overload',
               'cmp' => '_compare_overload',
               '""'  => '_stringify',
               '-'   => '_subtract_overload',
               '+'   => '_add_overload',
               'eq'  => '_string_equals_overload',
               'ne'  => '_string_not_equals_overload',
             );

# Have to load this after overloading is defined, after BEGIN blocks
# or else weird crashes ensue
require DateTime::Infinite;

use constant MAX_NANOSECONDS => 1_000_000_000;  # 1E9 = almost 32 bits

use constant INFINITY     =>      (9 ** 9 ** 9);
use constant NEG_INFINITY => -1 * (9 ** 9 ** 9);
use constant NAN          => INFINITY - INFINITY;

use constant SECONDS_PER_DAY => 86400;

use constant duration_class => 'DateTime::Duration';

my( @MonthLengths, @LeapYearMonthLengths );

BEGIN
{
    @MonthLengths =
        ( 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 );

    @LeapYearMonthLengths = @MonthLengths;
    $LeapYearMonthLengths[1]++;
}

{
    # I'd rather use Class::Data::Inheritable for this, but there's no
    # way to add the module-loading behavior to an accessor it
    # creates, despite what its docs say!
    my $DefaultLocale;
    sub DefaultLocale
    {
        my $class = shift;

        if (@_)
        {
            my $lang = shift;

            DateTime::Locale->load($lang);

            $DefaultLocale = $lang;
        }

        return $DefaultLocale;
    }
    # backwards compat
    *DefaultLanguage = \&DefaultLocale;
}
__PACKAGE__->DefaultLocale('en_US');

my $BasicValidate =
    { year   => { type => SCALAR,
                  callbacks =>
                  { 'is an integer' =>
                    sub { $_[0] =~ /^-?\d+$/ }
                  },
                },
      month  => { type => SCALAR, default => 1,
                  callbacks =>
                  { 'an integer between 1 and 12' =>
                    sub { $_[0] =~ /^\d+$/ && $_[0] >= 1 && $_[0] <= 12 }
                  },
                },
      day    => { type => SCALAR, default => 1,
                  callbacks =>
                  { 'an integer which is a possible valid day of month' =>
                    sub { $_[0] =~ /^\d+$/ && $_[0] >= 1 && $_[0] <= 31 }
                  },
                },
      hour   => { type => SCALAR, default => 0,
                  callbacks =>
                  { 'an integer between 0 and 23' =>
                    sub { $_[0] =~ /^\d+$/ && $_[0] >= 0 && $_[0] <= 23 },
                  },
                },
      minute => { type => SCALAR, default => 0,
                  callbacks =>
                  { 'an integer between 0 and 59' =>
                    sub { $_[0] =~ /^\d+$/ && $_[0] >= 0 && $_[0] <= 59 },
                  },
                },
      second => { type => SCALAR, default => 0,
                  callbacks =>
                  { 'an integer between 0 and 61' =>
                    sub { $_[0] =~ /^\d+$/ && $_[0] >= 0 && $_[0] <= 61 },
                  },
                },
      nanosecond => { type => SCALAR, default => 0,
                      callbacks =>
                      { 'a positive integer' =>
                        sub { $_[0] =~ /^\d+$/ && $_[0] >= 0 },
                      }
                    },
      locale    => { type => SCALAR | OBJECT,
                     default => undef },
      language  => { type => SCALAR | OBJECT,
                     optional => 1 },
    };

my $NewValidate =
    { %$BasicValidate,
      time_zone => { type => SCALAR | OBJECT,
                     default => 'floating' },
      formatter => { type => SCALAR | OBJECT, can => 'format_datetime', optional => 1 },
    };

sub new
{
    my $class = shift;
    my %p = validate( @_, $NewValidate );

    Carp::croak( "Invalid day of month (day = $p{day} - month = $p{month} - year = $p{year})\n" )
        if $p{day} > $class->_month_length( $p{year}, $p{month} );

    my $self = bless {}, $class;

    $p{locale} = delete $p{language} if exists $p{language};
    $p{locale} = $class->DefaultLocale unless defined $p{locale};

    if ( ref $p{locale} )
    {
        $self->{locale} = $p{locale};
    }
    else
    {
        $self->{locale} = DateTime::Locale->load( $p{locale} );
    }

    $self->{tz} =
        ( ref $p{time_zone} ?
          $p{time_zone} :
          DateTime::TimeZone->new( name => $p{time_zone} )
        );

    $self->{local_rd_days} =
        $class->_ymd2rd( @p{ qw( year month day ) } );

    $self->{local_rd_secs} =
        $class->_time_as_seconds( @p{ qw( hour minute second ) } );

    $self->{offset_modifier} = 0;

    $self->{rd_nanosecs} = $p{nanosecond};
    $self->{formatter} = $p{formatter};

    $self->_normalize_nanoseconds( $self->{local_rd_secs}, $self->{rd_nanosecs} );

    # Set this explicitly since it can't be calculated accurately
    # without knowing our time zone offset, and it's possible that the
    # offset can't be calculated without having at least a rough guess
    # of the datetime's year.  This year need not be correct, as long
    # as its equal or greater to the correct number, so we fudge by
    # adding one to the local year given to the constructor.
    $self->{utc_year} = $p{year} + 1;

    $self->_calc_utc_rd;

    $self->_handle_offset_modifier( $p{second} );

    $self->_calc_local_rd;

    if ( $p{second} > 59 )
    {
        if ( $self->{tz}->is_floating ||
             # If true, this means that the actual calculated leap
             # second does not occur in the second given to new()
             ( $self->{utc_rd_secs} - 86399
               <
               $p{second} - 59 )
           )
        {
            Carp::croak( "Invalid second value ($p{second})\n" );
        }
    }

    return $self;
}

# This method exists for the benefit of internal methods which create
# a new object based on the current object, like set() and truncate().
sub _new_from_self
{
    my $self = shift;

    my %old = map { $_ => $self->$_() }
        qw( year month day hour minute second nanosecond
            locale time_zone );
    $old{formatter} = $self->formatter()
        if defined $self->formatter();

    return (ref $self)->new( %old, @_ );
}

sub _handle_offset_modifier
{
    my $self = shift;

    $self->{offset_modifier} = 0;

    return if $self->{tz}->is_floating;

    my $second = shift;
    my $utc_is_valid = shift;

    my $utc_rd_days = $self->{utc_rd_days};

    my $offset = $utc_is_valid ? $self->offset : $self->_offset_for_local_datetime;

    if ( $offset >= 0
         && $self->{local_rd_secs} >= $offset
       )
    {
        if ( $second < 60 && $offset > 0 )
        {
            $self->{offset_modifier} =
                $self->_day_length( $utc_rd_days - 1 ) - SECONDS_PER_DAY;

            $self->{local_rd_secs} += $self->{offset_modifier};
        }
        elsif ( $second == 60
                &&
                ( ( $self->{local_rd_secs} == $offset
                    && $offset > 0 )
                  ||
                  ( $offset == 0
                    && $self->{local_rd_secs} > 86399 ) )
              )
        {
            my $mod = $self->_day_length( $utc_rd_days - 1 ) - SECONDS_PER_DAY;

            unless ( $mod == 0 )
            {
                $self->{utc_rd_secs} -= $mod;

                $self->_normalize_seconds;
            }
        }
    }
    elsif ( $offset < 0
            && $self->{local_rd_secs} >= SECONDS_PER_DAY + $offset )
    {
        if ( $second < 60 )
        {
            $self->{offset_modifier} =
                $self->_day_length( $utc_rd_days - 1 ) - SECONDS_PER_DAY;

            $self->{local_rd_secs} += $self->{offset_modifier};
        }
        elsif ( $second == 60 && $self->{local_rd_secs} == SECONDS_PER_DAY + $offset )
        {
            my $mod = $self->_day_length( $utc_rd_days - 1 ) - SECONDS_PER_DAY;

            unless ( $mod == 0 )
            {
                $self->{utc_rd_secs} -= $mod;

                $self->_normalize_seconds;
            }
        }
    }
}

sub _calc_utc_rd
{
    my $self = shift;

    delete $self->{utc_c};

    if ( $self->{tz}->is_utc || $self->{tz}->is_floating )
    {
        $self->{utc_rd_days} = $self->{local_rd_days};
        $self->{utc_rd_secs} = $self->{local_rd_secs};
    }
    else
    {
        my $offset = $self->_offset_for_local_datetime;

        $offset += $self->{offset_modifier};

        $self->{utc_rd_days} = $self->{local_rd_days};
        $self->{utc_rd_secs} = $self->{local_rd_secs} - $offset;
    }

    # We account for leap seconds in the new() method and nowhere else
    # except date math.
    $self->_normalize_tai_seconds( $self->{utc_rd_days}, $self->{utc_rd_secs} );
}

sub _normalize_seconds
{
    my $self = shift;

    return if $self->{utc_rd_secs} >= 0 && $self->{utc_rd_secs} <= 86399;

    if ( $self->{tz}->is_floating )
    {
        $self->_normalize_tai_seconds( $self->{utc_rd_days}, $self->{utc_rd_secs} );
    }
    else
    {
        $self->_normalize_leap_seconds( $self->{utc_rd_days}, $self->{utc_rd_secs} );
    }
}

sub _calc_local_rd
{
    my $self = shift;

    delete $self->{local_c};

    # We must short circuit for UTC times or else we could end up with
    # loops between DateTime.pm and DateTime::TimeZone
    if ( $self->{tz}->is_utc || $self->{tz}->is_floating )
    {
        $self->{local_rd_days} = $self->{utc_rd_days};
        $self->{local_rd_secs} = $self->{utc_rd_secs};
    }
    else
    {
        my $offset = $self->offset;

        $self->{local_rd_days} = $self->{utc_rd_days};
        $self->{local_rd_secs} = $self->{utc_rd_secs} + $offset;

        # intentionally ignore leap seconds here
        $self->_normalize_tai_seconds( $self->{local_rd_days}, $self->{local_rd_secs} );

        $self->{local_rd_secs} += $self->{offset_modifier};
    }

    $self->_calc_local_components;
}

sub _calc_local_components
{
    my $self = shift;

    @{ $self->{local_c} }{ qw( year month day day_of_week
                               day_of_year quarter day_of_quarter) } =
        $self->_rd2ymd( $self->{local_rd_days}, 1 );

    @{ $self->{local_c} }{ qw( hour minute second ) } =
        $self->_seconds_as_components
            ( $self->{local_rd_secs}, $self->{utc_rd_secs}, $self->{offset_modifier} );
}

sub _calc_utc_components
{
    my $self = shift;

    die "Cannot get UTC components before UTC RD has been calculated\n"
        unless defined $self->{utc_rd_days};

    @{ $self->{utc_c} }{ qw( year month day ) } =
        $self->_rd2ymd( $self->{utc_rd_days} );

    @{ $self->{utc_c} }{ qw( hour minute second ) } =
        $self->_seconds_as_components( $self->{utc_rd_secs} );
}

sub _utc_ymd
{
    my $self = shift;

    $self->_calc_utc_components unless exists $self->{utc_c}{year};

    return @{ $self->{utc_c} }{ qw( year month day ) };
}

sub _utc_hms
{
    my $self = shift;

    $self->_calc_utc_components unless exists $self->{utc_c}{hour};

    return @{ $self->{utc_c} }{ qw( hour minute second ) };
}

{
    my $spec = { epoch      => { regex => qr/^-?(?:\d+(?:\.\d*)?|\.\d+)$/ },
                 locale     => { type => SCALAR | OBJECT, optional => 1 },
                 language   => { type => SCALAR | OBJECT, optional => 1 },
                 time_zone  => { type => SCALAR | OBJECT, optional => 1 },
                 formatter  => { type => SCALAR | OBJECT, can => 'format_datetime',
                                 optional => 1 },
               };

    sub from_epoch
    {
        my $class = shift;
        my %p = validate( @_, $spec );

        my %args;

        # Because epoch may come from Time::HiRes
        my $fraction = $p{epoch} - int( $p{epoch} );
        $args{nanosecond} = int( $fraction * MAX_NANOSECONDS )
            if $fraction;

        # Note, for very large negative values this may give a
        # blatantly wrong answer.
        @args{ qw( second minute hour day month year ) } =
            ( gmtime( int delete $p{epoch} ) )[ 0..5 ];
        $args{year} += 1900;
        $args{month}++;

        my $self = $class->new( %p, %args, time_zone => 'UTC' );

        $self->set_time_zone( $p{time_zone} ) if exists $p{time_zone};

        return $self;
    }
}

# use scalar time in case someone's loaded Time::Piece
sub now { shift->from_epoch( epoch => (scalar time), @_ ) }

sub today { shift->now(@_)->truncate( to => 'day' ) }

{
    my $spec = { object => { type => OBJECT,
                             can => 'utc_rd_values',
                           },
                 locale     => { type => SCALAR | OBJECT, optional => 1 },
                 language   => { type => SCALAR | OBJECT, optional => 1 },
                 formatter  => { type => SCALAR | OBJECT, can => 'format_datetime',
                                 optional => 1 },
               };

    sub from_object
    {
        my $class = shift;
        my %p = validate( @_, $spec );

        my $object = delete $p{object};

        my ( $rd_days, $rd_secs, $rd_nanosecs ) = $object->utc_rd_values;

        # A kludge because until all calendars are updated to return all
        # three values, $rd_nanosecs could be undef
        $rd_nanosecs ||= 0;

        # This is a big hack to let _seconds_as_components operate naively
        # on the given value.  If the object _is_ on a leap second, we'll
        # add that to the generated seconds value later.
        my $leap_seconds = 0;
        if ( $object->can('time_zone') && ! $object->time_zone->is_floating
             && $rd_secs > 86399 && $rd_secs <= $class->_day_length($rd_days) )
        {
            $leap_seconds = $rd_secs - 86399;
            $rd_secs -= $leap_seconds;
        }

        my %args;
        @args{ qw( year month day ) } = $class->_rd2ymd($rd_days);
        @args{ qw( hour minute second ) } =
            $class->_seconds_as_components($rd_secs);
        $args{nanosecond} = $rd_nanosecs;

        $args{second} += $leap_seconds;

        my $new = $class->new( %p, %args, time_zone => 'UTC' );

        if ( $object->can('time_zone') )
        {
            $new->set_time_zone( $object->time_zone );
        }
        else
        {
            $new->set_time_zone( 'floating' );
        }

        return $new;
    }
}

my $LastDayOfMonthValidate = { %$NewValidate };
foreach ( keys %$LastDayOfMonthValidate )
{
    my %copy = %{ $LastDayOfMonthValidate->{$_} };

    delete $copy{default};
    $copy{optional} = 1 unless $_ eq 'year' || $_ eq 'month';

    $LastDayOfMonthValidate->{$_} = \%copy;
}

sub last_day_of_month
{
    my $class = shift;
    my %p = validate( @_, $LastDayOfMonthValidate );

    my $day = $class->_month_length( $p{year}, $p{month} );

    return $class->new( %p, day => $day );
}

sub _month_length
{
    return ( $_[0]->_is_leap_year( $_[1] ) ?
             $LeapYearMonthLengths[ $_[2] - 1 ] :
             $MonthLengths[ $_[2] - 1 ]
           );
}

my $FromDayOfYearValidate = { %$NewValidate };
foreach ( keys %$FromDayOfYearValidate )
{
    next if $_ eq 'month' || $_ eq 'day';

    my %copy = %{ $FromDayOfYearValidate->{$_} };

    delete $copy{default};
    $copy{optional} = 1 unless $_ eq 'year' || $_ eq 'month';

    $FromDayOfYearValidate->{$_} = \%copy;
}
$FromDayOfYearValidate->{day_of_year} =
    { type => SCALAR,
      callbacks =>
      { 'is between 1 and 366' =>
        sub { $_[0] >= 1 && $_[0] <= 366 }
      }
    };
sub from_day_of_year
{
    my $class = shift;
    my %p = validate( @_, $FromDayOfYearValidate );

    my $is_leap_year = $class->_is_leap_year( $p{year} );

    Carp::croak( "$p{year} is not a leap year.\n" )
        if $p{day_of_year} == 366 && ! $is_leap_year;

    my $month = 1;
    my $day = delete $p{day_of_year};

    while ( $month <= 12 && $day > $class->_month_length( $p{year}, $month ) )
    {
        $day -= $class->_month_length( $p{year}, $month );
        $month++;
    }

    return DateTime->new( %p,
                          month => $month,
                          day   => $day,
                        );
}

sub formatter { $_[0]->{formatter} }

sub clone { bless { %{ $_[0] } }, ref $_[0] }

sub year {
    Carp::carp('year() is a read-only accessor') if @_ > 1;
    return $_[0]->{local_c}{year};
}

sub ce_year { $_[0]->{local_c}{year} <= 0 ?
              $_[0]->{local_c}{year} - 1 :
              $_[0]->{local_c}{year} }

sub era_name { $_[0]->{locale}->era_wide->[ $_[0]->_era_index() ] }

sub era_abbr { $_[0]->{locale}->era_abbreviated->[ $_[0]->_era_index() ] }
# deprecated
*era = \&era_abbr;

sub _era_index { $_[0]->{local_c}{year} <= 0 ? 0 : 1 }

sub christian_era { $_[0]->ce_year > 0 ? 'AD' : 'BC' }
sub secular_era   { $_[0]->ce_year > 0 ? 'CE' : 'BCE' }

sub year_with_era { (abs $_[0]->ce_year) . $_[0]->era_abbr }
sub year_with_christian_era { (abs $_[0]->ce_year) . $_[0]->christian_era }
sub year_with_secular_era   { (abs $_[0]->ce_year) . $_[0]->secular_era }

sub month   {
    Carp::carp('month() is a read-only accessor') if @_ > 1;
    return $_[0]->{local_c}{month};
}
*mon = \&month;

sub month_0 { $_[0]->{local_c}{month} - 1 }
*mon_0 = \&month_0;

sub month_name { $_[0]->{locale}->month_format_wide->[ $_[0]->month_0() ] }

sub month_abbr { $_[0]->{locale}->month_format_abbreviated->[ $_[0]->month_0() ] }

sub day_of_month {
    Carp::carp('day_of_month() is a read-only accessor') if @_ > 1;
    $_[0]->{local_c}{day};
}
*day  = \&day_of_month;
*mday = \&day_of_month;

sub weekday_of_month { use integer; ( ( $_[0]->day - 1 ) / 7 ) + 1 }

sub quarter { $_[0]->{local_c}{quarter} }

sub quarter_name { $_[0]->{locale}->quarter_format_wide->[ $_[0]->quarter_0() ] }
sub quarter_abbr { $_[0]->{locale}->quarter_format_abbreviated->[ $_[0]->quarter_0() ] }

sub quarter_0 { $_[0]->{local_c}{quarter} - 1 }

sub day_of_month_0 { $_[0]->{local_c}{day} - 1 }
*day_0  = \&day_of_month_0;
*mday_0 = \&day_of_month_0;

sub day_of_week { $_[0]->{local_c}{day_of_week} }
*wday = \&day_of_week;
*dow  = \&day_of_week;

sub day_of_week_0 { $_[0]->{local_c}{day_of_week} - 1 }
*wday_0 = \&day_of_week_0;
*dow_0  = \&day_of_week_0;

sub local_day_of_week
{
    my $self = shift;

    my $day = $self->day_of_week();

    my $local_first_day = $self->{locale}->first_day_of_week();

    my $d = ( ( 8 - $local_first_day ) + $day ) % 7;

    return $d == 0 ? 7 : $d;
}

sub day_name { $_[0]->{locale}->day_format_wide->[ $_[0]->day_of_week_0() ] }

sub day_abbr { $_[0]->{locale}->day_format_abbreviated->[ $_[0]->day_of_week_0() ] }

sub day_of_quarter { $_[0]->{local_c}{day_of_quarter} }
*doq = \&day_of_quarter;

sub day_of_quarter_0 { $_[0]->day_of_quarter - 1 }
*doq_0 = \&day_of_quarter_0;

sub day_of_year { $_[0]->{local_c}{day_of_year} }
*doy = \&day_of_year;

sub day_of_year_0 { $_[0]->{local_c}{day_of_year} - 1 }
*doy_0 = \&day_of_year_0;

sub am_or_pm { $_[0]->{locale}->am_pm_abbreviated->[ $_[0]->hour() < 12 ? 0 : 1 ] }

sub ymd
{
    my ( $self, $sep ) = @_;
    $sep = '-' unless defined $sep;

    return sprintf( "%0.4d%s%0.2d%s%0.2d",
                    $self->year, $sep,
                    $self->{local_c}{month}, $sep,
                    $self->{local_c}{day} );
}
*date = \&ymd;

sub mdy
{
    my ( $self, $sep ) = @_;
    $sep = '-' unless defined $sep;

    return sprintf( "%0.2d%s%0.2d%s%0.4d",
                    $self->{local_c}{month}, $sep,
                    $self->{local_c}{day}, $sep,
                    $self->year );
}

sub dmy
{
    my ( $self, $sep ) = @_;
    $sep = '-' unless defined $sep;

    return sprintf( "%0.2d%s%0.2d%s%0.4d",
                    $self->{local_c}{day}, $sep,
                    $self->{local_c}{month}, $sep,
                    $self->year );
}

sub hour   {
    Carp::carp('hour() is a read-only accessor') if @_ > 1;
    return $_[0]->{local_c}{hour};
}
sub hour_1 { $_[0]->{local_c}{hour} == 0 ? 24 : $_[0]->{local_c}{hour} }

sub hour_12   { my $h = $_[0]->hour % 12; return $h ? $h : 12 }
sub hour_12_0 { $_[0]->hour % 12 }

sub minute {
    Carp::carp('minute() is a read-only accessor') if @_ > 1;
    return $_[0]->{local_c}{minute};
}
*min = \&minute;

sub second {
    Carp::carp('second() is a read-only accessor') if @_ > 1;
    return $_[0]->{local_c}{second};
}
*sec = \&second;

sub fractional_second { $_[0]->second + $_[0]->nanosecond / MAX_NANOSECONDS }

sub nanosecond {
    Carp::carp('nanosecond() is a read-only accessor') if @_ > 1;
    return $_[0]->{rd_nanosecs};
}

sub millisecond { _round( $_[0]->{rd_nanosecs} / 1000000 ) }

sub microsecond { _round( $_[0]->{rd_nanosecs} / 1000 ) }

sub _round
{
    my $val = shift;
    my $int = int $val;

    return $val - $int >= 0.5 ? $int + 1 : $int;
}

sub leap_seconds
{
    my $self = shift;

    return 0 if $self->{tz}->is_floating;

    return DateTime->_accumulated_leap_seconds( $self->{utc_rd_days} );
}

sub _stringify
{
    my $self = shift;

    return $self->iso8601 unless $self->{formatter};
    return $self->{formatter}->format_datetime($self);
}

sub hms
{
    my ( $self, $sep ) = @_;
    $sep = ':' unless defined $sep;

    return sprintf( "%0.2d%s%0.2d%s%0.2d",
                    $self->{local_c}{hour}, $sep,
                    $self->{local_c}{minute}, $sep,
                    $self->{local_c}{second} );
}
# don't want to override CORE::time()
*DateTime::time = \&hms;

sub iso8601 { join 'T', $_[0]->ymd('-'), $_[0]->hms(':') }
*datetime = \&iso8601;

sub is_leap_year { $_[0]->_is_leap_year( $_[0]->year ) }

sub week
{
    my $self = shift;

    unless ( defined $self->{local_c}{week_year} )
    {
        # This algorithm was taken from Date::Calc's DateCalc.c file
        my $jan_one_dow_m1 =
            ( ( $self->_ymd2rd( $self->year, 1, 1 ) + 6 ) % 7 );

        $self->{local_c}{week_number} =
            int( ( ( $self->day_of_year - 1 ) + $jan_one_dow_m1 ) / 7 );
        $self->{local_c}{week_number}++ if $jan_one_dow_m1 < 4;

        if ( $self->{local_c}{week_number} == 0 )
        {
            $self->{local_c}{week_year} = $self->year - 1;
            $self->{local_c}{week_number} =
                $self->_weeks_in_year( $self->{local_c}{week_year} );
        }
        elsif ( $self->{local_c}{week_number} == 53 &&
                $self->_weeks_in_year( $self->year ) == 52 )
        {
            $self->{local_c}{week_number} = 1;
            $self->{local_c}{week_year} = $self->year + 1;
        }
        else
        {
            $self->{local_c}{week_year} = $self->year;
        }
    }

    return @{ $self->{local_c} }{ 'week_year', 'week_number' }
}

# Also from DateCalc.c
sub _weeks_in_year
{
    my $self = shift;
    my $year = shift;

    my $jan_one_dow =
        ( ( $self->_ymd2rd( $year, 1, 1 ) + 6 ) % 7 ) + 1;
    my $dec_31_dow =
        ( ( $self->_ymd2rd( $year, 12, 31 ) + 6 ) % 7 ) + 1;

    return $jan_one_dow == 4 || $dec_31_dow == 4 ? 53 : 52;
}

sub week_year   { ($_[0]->week)[0] }
sub week_number { ($_[0]->week)[1] }

# ISO says that the first week of a year is the first week containing
# a Thursday.  Extending that says that the first week of the month is
# the first week containing a Thursday.  ICU agrees.
#
# Algorithm supplied by Rick Measham, who doesn't understand how it
# works.  Neither do I.  Please feel free to explain this to me!
sub week_of_month
{
    my $self = shift;

    # Faster than cloning just to get the dow
    my $first_wday_of_month = ( 8 - ( $self->day - $self->dow ) % 7 ) % 7;
    $first_wday_of_month = 7 unless $first_wday_of_month;

    my $wom = int( ( $self->day + $first_wday_of_month - 2 ) / 7 );
    return ( $first_wday_of_month <= 4 ) ? $wom + 1 : $wom;
}

sub time_zone {
    Carp::carp('time_zone() is a read-only accessor') if @_ > 1;
    return $_[0]->{tz};
}

sub offset                     { $_[0]->{tz}->offset_for_datetime( $_[0] ) }
sub _offset_for_local_datetime { $_[0]->{tz}->offset_for_local_datetime( $_[0] ) }

sub is_dst { $_[0]->{tz}->is_dst_for_datetime( $_[0] ) }

sub time_zone_long_name  { $_[0]->{tz}->name }
sub time_zone_short_name { $_[0]->{tz}->short_name_for_datetime( $_[0] ) }

sub locale {
    Carp::carp('locale() is a read-only accessor') if @_ > 1;
    return $_[0]->{locale};
}
*language = \&locale;

sub utc_rd_values { @{ $_[0] }{ 'utc_rd_days', 'utc_rd_secs', 'rd_nanosecs' } }
sub local_rd_values { @{ $_[0] }{ 'local_rd_days', 'local_rd_secs', 'rd_nanosecs' } }

# NOTE: no nanoseconds, no leap seconds
sub utc_rd_as_seconds   { ( $_[0]->{utc_rd_days} * SECONDS_PER_DAY ) + $_[0]->{utc_rd_secs} }

# NOTE: no nanoseconds, no leap seconds
sub local_rd_as_seconds { ( $_[0]->{local_rd_days} * SECONDS_PER_DAY ) + $_[0]->{local_rd_secs} }

# RD 1 is JD 1,721,424.5 - a simple offset
sub jd
{
    my $self = shift;

    my $jd = $self->{utc_rd_days} + 1_721_424.5;

    my $day_length = $self->_day_length( $self->{utc_rd_days} );

    return ( $jd +
             ( $self->{utc_rd_secs} / $day_length )  +
             ( $self->{rd_nanosecs} / $day_length / MAX_NANOSECONDS )
           );
}

sub mjd { $_[0]->jd - 2_400_000.5 }

{
    my %strftime_patterns =
        ( 'a' => sub { $_[0]->day_abbr },
          'A' => sub { $_[0]->day_name },
          'b' => sub { $_[0]->month_abbr },
          'B' => sub { $_[0]->month_name },
          'c' => sub { $_[0]->format_cldr( $_[0]->{locale}->datetime_format_default() ) },
          'C' => sub { int( $_[0]->year / 100 ) },
          'd' => sub { sprintf( '%02d', $_[0]->day_of_month ) },
          'D' => sub { $_[0]->strftime( '%m/%d/%y' ) },
          'e' => sub { sprintf( '%2d', $_[0]->day_of_month ) },
          'F' => sub { $_[0]->ymd('-') },
          'g' => sub { substr( $_[0]->week_year, -2 ) },
          'G' => sub { $_[0]->week_year },
          'H' => sub { sprintf( '%02d', $_[0]->hour ) },
          'I' => sub { sprintf( '%02d', $_[0]->hour_12 ) },
          'j' => sub { $_[0]->day_of_year },
          'k' => sub { sprintf( '%2d', $_[0]->hour ) },
          'l' => sub { sprintf( '%2d', $_[0]->hour_12 ) },
          'm' => sub { sprintf( '%02d', $_[0]->month ) },
          'M' => sub { sprintf( '%02d', $_[0]->minute ) },
          'n' => sub { "\n" }, # should this be OS-sensitive?
          'N' => \&_format_nanosecs,
          'p' => sub { $_[0]->am_or_pm() },
          'P' => sub { lc $_[0]->am_or_pm() },
          'r' => sub { $_[0]->strftime( '%I:%M:%S %p' ) },
          'R' => sub { $_[0]->strftime( '%H:%M' ) },
          's' => sub { $_[0]->epoch },
          'S' => sub { sprintf( '%02d', $_[0]->second ) },
          't' => sub { "\t" },
          'T' => sub { $_[0]->strftime( '%H:%M:%S' ) },
          'u' => sub { $_[0]->day_of_week },
          # algorithm from Date::Format::wkyr
          'U' => sub { my $dow = $_[0]->day_of_week;
                       $dow = 0 if $dow == 7; # convert to 0-6, Sun-Sat
                       my $doy = $_[0]->day_of_year - 1;
                       return sprintf( '%02d', int( ( $doy - $dow + 13 ) / 7 - 1 ) )
                   },
          'V' => sub { sprintf( '%02d', $_[0]->week_number ) },
          'w' => sub { my $dow = $_[0]->day_of_week;
                       return $dow % 7;
                   },
          'W' => sub { my $dow = $_[0]->day_of_week;
                       my $doy = $_[0]->day_of_year - 1;
                       return sprintf( '%02d', int( ( $doy - $dow + 13 ) / 7 - 1 ) )
                   },
          'x' => sub { $_[0]->format_cldr( $_[0]->{locale}->date_format_default() ) },
          'X' => sub { $_[0]->format_cldr( $_[0]->{locale}->time_format_default() ) },
          'y' => sub { sprintf( '%02d', substr( $_[0]->year, -2 ) ) },
          'Y' => sub { return $_[0]->year },
          'z' => sub { DateTime::TimeZone->offset_as_string( $_[0]->offset ) },
          'Z' => sub { $_[0]->{tz}->short_name_for_datetime( $_[0] ) },
          '%' => sub { '%' },
        );

    $strftime_patterns{h} = $strftime_patterns{b};

    sub strftime
    {
        my $self = shift;
        # make a copy or caller's scalars get munged
        my @patterns = @_;

        my @r;
        foreach my $p (@patterns)
        {
            $p =~ s/
                    (?:
                      %{(\w+)}         # method name like %{day_name}
                      |
                      %([%a-zA-Z])     # single character specifier like %d
                      |
                      %(\d+)N          # special case for %N
                    )
                   /
                    ( $1
                      ? ( $self->can($1) ? $self->$1() : "\%{$1}" )
                      : $2
                      ? ( $strftime_patterns{$2} ? $strftime_patterns{$2}->($self) : "\%$2" )
                      : $3
                      ? $strftime_patterns{N}->($self, $3)
                      : ''  # this won't happen
                    )
                   /sgex;

            return $p unless wantarray;

            push @r, $p;
        }

        return @r;
    }
}

{
    # It's an array because the order in which the regexes are checked
    # is important. These patterns are similar to the ones Java uses,
    # but not quite the same. See
    # http://www.unicode.org/reports/tr35/tr35-9.html#Date_Format_Patterns.
    my @patterns =
        ( qr/GGGGG/  => sub { $_[0]->{locale}->era_narrow->[ $_[0]->_era_index() ] },
          qr/GGGG/   => 'era_name',
          qr/G{1,3}/ => 'era_abbr',

          qr/(y{3,5})/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->year() ) },
          # yy is a weird special case, where it must be exactly 2 digits
          qr/yy/       => sub { my $year = $_[0]->year();
                                my $y2 = substr( $year, -2, 2 ) if length $year > 2;
                                $y2 *= -1 if $year < 0;
                                $_[0]->_zero_padded_number( 'yy', $y2 ) },
          qr/y/        => sub { $_[0]->year() },
          qr/(u+)/     => sub { $_[0]->_zero_padded_number( $1, $_[0]->year() ) },
          qr/(Y+)/     => sub { $_[0]->_zero_padded_number( $1, $_[0]->week_year() ) },

          qr/QQQQ/  => 'quarter_name',
          qr/QQQ/   => 'quarter_abbr',
          qr/(QQ?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->quarter() ) },

          qr/qqqq/  => sub { $_[0]->{locale}->quarter_stand_alone_wide()->[ $_[0]->quarter_0() ] },
          qr/qqq/   => sub { $_[0]->{locale}->quarter_stand_alone_abbreviated()->[ $_[0]->quarter_0() ] },
          qr/(qq?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->quarter() ) },

          qr/MMMMM/ => sub { $_[0]->{locale}->month_format_narrow->[ $_[0]->month_0() ] },
          qr/MMMM/  => 'month_name',
          qr/MMM/   => 'month_abbr',
          qr/(MM?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->month() ) },

          qr/LLLLL/ => sub { $_[0]->{locale}->month_stand_alone_narrow->[ $_[0]->month_0() ] },
          qr/LLLL/  => sub { $_[0]->{locale}->month_stand_alone_wide->[ $_[0]->month_0() ] },
          qr/LLL/   => sub { $_[0]->{locale}->month_stand_alone_abbreviated->[ $_[0]->month_0() ] },
          qr/(LL?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->month() ) },

          qr/(ww?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->week_number() ) },
          qr/W/     => 'week_of_month',

          qr/(dd?)/    => sub { $_[0]->_zero_padded_number( $1, $_[0]->day_of_month() ) },
          qr/(D{1,3})/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->day_of_year() ) },

          qr/F/    => 'weekday_of_month',
          qr/(g+)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->mjd() ) },

          qr/EEEEE/  => sub { $_[0]->{locale}->day_format_narrow->[ $_[0]->day_of_week_0() ] },
          qr/EEEE/   => 'day_name',
          qr/E{1,3}/ => 'day_abbr',

          qr/eeeee/ => sub { $_[0]->{locale}->day_format_narrow->[ $_[0]->day_of_week_0() ] },
          qr/eeee/  => 'day_name',
          qr/eee/   => 'day_abbr',
          qr/(ee?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->local_day_of_week() ) },

          qr/ccccc/ => sub { $_[0]->{locale}->day_stand_alone_narrow->[ $_[0]->day_of_week_0() ] },
          qr/cccc/  => sub { $_[0]->{locale}->day_stand_alone_wide->[ $_[0]->day_of_week_0() ] },
          qr/ccc/   => sub { $_[0]->{locale}->day_stand_alone_abbreviated->[ $_[0]->day_of_week_0() ] },
          qr/(cc?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->day_of_week() ) },

          qr/a/ => 'am_or_pm',

          qr/(hh?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->hour_12() ) },
          qr/(HH?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->hour() ) },
          qr/(KK?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->hour_12_0() ) },
          qr/(kk?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->hour_1() ) },
          qr/(jj?)/ => sub { my $h = $_[0]->{locale}->prefers_24_hour_time() ? $_[0]->hour() : $_[0]->hour_12();
                             $_[0]->_zero_padded_number( $1, $h ) },

          qr/(mm?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->minute() ) },

          qr/(ss?)/ => sub { $_[0]->_zero_padded_number( $1, $_[0]->second() ) },
          # I'm not sure this is what is wanted (notably the trailing
          # and leading zeros it can produce), but once again the LDML
          # spec is not all that clear.
          qr/(S+)/  => sub { my $l = length $1;
                             my $val = sprintf( "%.${l}f", $_[0]->fractional_second() - $_[0]->second() );
                             $val =~ s/^0\.//;
                             $val || 0 },
          qr/A+/    => sub { ( $_[0]->{local_rd_secs} * 1000 ) + $_[0]->millisecond() },

          qr/zzzz/   => sub { $_[0]->time_zone_long_name() },
          qr/z{1,3}/ => sub { $_[0]->time_zone_short_name() },
          qr/ZZZZ/   => sub { $_[0]->time_zone_short_name()
                              . DateTime::TimeZone->offset_as_string( $_[0]->offset() ) },
          qr/Z{1,3}/ => sub { DateTime::TimeZone->offset_as_string( $_[0]->offset() ) },
          qr/vvvv/   => sub { $_[0]->time_zone_long_name() },
          qr/v{1,3}/ => sub { $_[0]->time_zone_short_name() },
          qr/VVVV/   => sub { $_[0]->time_zone_long_name() },
          qr/V{1,3}/ => sub { $_[0]->time_zone_short_name() },
    );

    sub _zero_padded_number
    {
        my $self = shift;
        my $size = length shift;
        my $val  = shift;

        return sprintf( "%0${size}d", $val );
    }

    sub _space_padded_string
    {
        my $self = shift;
        my $size = length shift;
        my $val  = shift;

        return sprintf( "% ${size}s", $val );
    }

    sub format_cldr
    {
        my $self = shift;
        # make a copy or caller's scalars get munged
        my @patterns = @_;

        my @r;
        foreach my $p (@patterns)
        {
            $p =~ s/\G
                    (?:
                      '((?:[^']|'')*)' # quote escaped bit of text
                                       # it needs to end with one
                                       # quote not followed by
                                       # another
                      |
                      (([a-zA-Z])\3*)     # could be a pattern
                      |
                      (.)                 # anything else
                    )
                   /
                    defined $1
                    ? $1
                    : defined $2
                    ? $self->_cldr_pattern($2)
                    : defined $4
                    ? $4
                    : undef # should never get here
                   /sgex;

            $p =~ s/\'\'/\'/g;

            return $p unless wantarray;

            push @r, $p;
        }

        return @r;
    }

    sub _cldr_pattern
    {
        my $self    = shift;
        my $pattern = shift;

        for ( my $i = 0; $i < @patterns; $i +=2 )
        {
            if ( $pattern =~ /$patterns[$i]/ )
            {
                my $sub = $patterns[ $i + 1 ];

                return $self->$sub();
            }
        }

        return $pattern;
    }
}

sub _format_nanosecs
{
    my $self = shift;
    my $precision = shift;

    my $ret = sprintf( "%09d", $self->{rd_nanosecs} );
    return $ret unless $precision;   # default = 9 digits

    # rd_nanosecs might contain a fractional separator
    my ( $int, $frac ) = split /[.,]/, $self->{rd_nanosecs};
    $ret .= $frac if $frac;

    return substr( $ret, 0, $precision );
}

sub epoch
{
    my $self = shift;

    return $self->{utc_c}{epoch}
        if exists $self->{utc_c}{epoch};

    my ( $year, $month, $day ) = $self->_utc_ymd;
    my @hms = $self->_utc_hms;

    $self->{utc_c}{epoch} =
        timegm_nocheck( ( reverse @hms ),
                        $day,
                        $month - 1,
                        $year,
                      );

    return $self->{utc_c}{epoch};
}

sub hires_epoch
{
    my $self = shift;

    my $epoch = $self->epoch;

    return undef unless defined $epoch;

    my $nano = $self->{rd_nanosecs} / MAX_NANOSECONDS;

    return $epoch + $nano;
}

sub is_finite { 1 }
sub is_infinite { 0 }

# added for benefit of DateTime::TimeZone
sub utc_year { $_[0]->{utc_year} }

# returns a result that is relative to the first datetime
sub subtract_datetime
{
    my $dt1 = shift;
    my $dt2 = shift;

    $dt2 = $dt2->clone->set_time_zone( $dt1->time_zone )
        unless $dt1->time_zone eq $dt2->time_zone;

    # We only want a negative duration if $dt2 > $dt1 ($self)
    my ( $bigger, $smaller, $negative ) =
        ( $dt1 >= $dt2 ?
          ( $dt1, $dt2, 0 ) :
          ( $dt2, $dt1, 1 )
        );

    my $is_floating = $dt1->time_zone->is_floating &&
                      $dt2->time_zone->is_floating;


    my $minute_length = 60;
    unless ($is_floating)
    {
        my ( $utc_rd_days, $utc_rd_secs ) = $smaller->utc_rd_values;

        if ( $utc_rd_secs >= 86340 && ! $is_floating )
        {
            # If the smaller of the two datetimes occurs in the last
            # UTC minute of the UTC day, then that minute may not be
            # 60 seconds long.  If we need to subtract a minute from
            # the larger datetime's minutes count in order to adjust
            # the seconds difference to be positive, we need to know
            # how long that minute was.  If one of the datetimes is
            # floating, we just assume a minute is 60 seconds.

            $minute_length = $dt1->_day_length($utc_rd_days) - 86340;
        }
    }

    # This is a gross hack that basically figures out if the bigger of
    # the two datetimes is the day of a DST change.  If it's a 23 hour
    # day (switching _to_ DST) then we subtract 60 minutes from the
    # local time.  If it's a 25 hour day then we add 60 minutes to the
    # local time.
    #
    # This produces the most "intuitive" results, though there are
    # still reversibility problems with the resultant duration.
    #
    # However, if the two objects are on the same (local) date, and we
    # are not crossing a DST change, we don't want to invoke the hack
    # - see 38local-subtract.t
    my $bigger_min = $bigger->hour * 60 + $bigger->minute;
    if ( $bigger->time_zone->has_dst_changes
         && $bigger->is_dst != $smaller->is_dst
       )
    {

        $bigger_min -= 60
            # it's a 23 hour (local) day
            if ( $bigger->is_dst
                 &&
                 do { local $@;
                      my $prev_day = eval { $bigger->clone->subtract( days => 1 ) };
                      $prev_day && ! $prev_day->is_dst ? 1 : 0 }
               );

        $bigger_min += 60
            # it's a 25 hour (local) day
            if ( ! $bigger->is_dst
                 &&
                 do { local $@;
                      my $prev_day = eval { $bigger->clone->subtract( days => 1 ) };
                      $prev_day && $prev_day->is_dst ? 1 : 0 }
               );
    }

    my ( $months, $days, $minutes, $seconds, $nanoseconds ) =
        $dt1->_adjust_for_positive_difference
            ( $bigger->year * 12 + $bigger->month, $smaller->year * 12 + $smaller->month,

              $bigger->day, $smaller->day,

              $bigger_min, $smaller->hour * 60 + $smaller->minute,

	      $bigger->second, $smaller->second,

	      $bigger->nanosecond, $smaller->nanosecond,

	      $minute_length,

              # XXX - using the smaller as the month length is
              # somewhat arbitrary, we could also use the bigger -
              # either way we have reversibility problems
	      $dt1->_month_length( $smaller->year, $smaller->month ),
            );

    if ($negative)
    {
        for ( $months, $days, $minutes, $seconds, $nanoseconds )
        {
	    # Some versions of Perl can end up with -0 if we do "0 * -1"!!
            $_ *= -1 if $_;
        }
    }

    return
        $dt1->duration_class->new
            ( months      => $months,
	      days        => $days,
	      minutes     => $minutes,
              seconds     => $seconds,
              nanoseconds => $nanoseconds,
            );
}

sub _adjust_for_positive_difference
{
    my ( $self,
	 $month1, $month2,
	 $day1, $day2,
	 $min1, $min2,
	 $sec1, $sec2,
	 $nano1, $nano2,
	 $minute_length,
	 $month_length,
       ) = @_;

    if ( $nano1 < $nano2 )
    {
        $sec1--;
        $nano1 += MAX_NANOSECONDS;
    }

    if ( $sec1 < $sec2 )
    {
        $min1--;
        $sec1 += $minute_length;
    }

    # A day always has 24 * 60 minutes, though the minutes may vary in
    # length.
    if ( $min1 < $min2 )
    {
	$day1--;
	$min1 += 24 * 60;
    }

    if ( $day1 < $day2 )
    {
	$month1--;
	$day1 += $month_length;
    }

    return ( $month1 - $month2,
	     $day1 - $day2,
	     $min1 - $min2,
             $sec1 - $sec2,
             $nano1 - $nano2,
           );
}

sub subtract_datetime_absolute
{
    my $self = shift;
    my $dt = shift;

    my $utc_rd_secs1 = $self->utc_rd_as_seconds;
    $utc_rd_secs1 += DateTime->_accumulated_leap_seconds( $self->{utc_rd_days} )
	if ! $self->time_zone->is_floating;

    my $utc_rd_secs2 = $dt->utc_rd_as_seconds;
    $utc_rd_secs2 += DateTime->_accumulated_leap_seconds( $dt->{utc_rd_days} )
	if ! $dt->time_zone->is_floating;

    my $seconds = $utc_rd_secs1 - $utc_rd_secs2;
    my $nanoseconds = $self->nanosecond - $dt->nanosecond;

    if ( $nanoseconds < 0 )
    {
	$seconds--;
	$nanoseconds += MAX_NANOSECONDS;
    }

    return
        $self->duration_class->new
            ( seconds     => $seconds,
              nanoseconds => $nanoseconds,
            );
}

sub delta_md
{
    my $self = shift;
    my $dt = shift;

    my ( $smaller, $bigger ) = sort $self, $dt;

    my ( $months, $days, undef, undef, undef ) =
        $dt->_adjust_for_positive_difference
            ( $bigger->year * 12 + $bigger->month, $smaller->year * 12 + $smaller->month,

              $bigger->day, $smaller->day,

              0, 0,

              0, 0,

              0, 0,

	      60,

	      $smaller->_month_length( $smaller->year, $smaller->month ),
            );

    return $self->duration_class->new( months => $months,
                                       days   => $days );
}

sub delta_days
{
    my $self = shift;
    my $dt = shift;

    my $days = abs( ($self->local_rd_values)[0] - ($dt->local_rd_values)[0] );

    $self->duration_class->new( days => $days );
}

sub delta_ms
{
    my $self = shift;
    my $dt = shift;

    my ( $smaller, $greater ) = sort $self, $dt;

    my $days = int( $greater->jd - $smaller->jd );

    my $dur = $greater->subtract_datetime($smaller);

    my %p;
    $p{hours}   = $dur->hours + ( $days * 24 );
    $p{minutes} = $dur->minutes;
    $p{seconds} = $dur->seconds;

    return $self->duration_class->new(%p);
}

sub _add_overload
{
    my ( $dt, $dur, $reversed ) = @_;

    if ($reversed)
    {
        ( $dur, $dt ) = ( $dt, $dur );
    }

    unless ( DateTime::Helpers::isa( $dur, 'DateTime::Duration' ) )
    {
        my $class = ref $dt;
        my $dt_string = overload::StrVal($dt);

        Carp::croak( "Cannot add $dur to a $class object ($dt_string).\n"
                     . " Only a DateTime::Duration object can "
                     . " be added to a $class object." );
    }

    return $dt->clone->add_duration($dur);
}

sub _subtract_overload
{
    my ( $date1, $date2, $reversed ) = @_;

    if ($reversed)
    {
        ( $date2, $date1 ) = ( $date1, $date2 );
    }

    if ( DateTime::Helpers::isa( $date2, 'DateTime::Duration' ) )
    {
        my $new = $date1->clone;
        $new->add_duration( $date2->inverse );
        return $new;
    }
    elsif ( DateTime::Helpers::isa( $date2, 'DateTime' ) )
    {
        return $date1->subtract_datetime($date2);
    }
    else
    {
        my $class = ref $date1;
        my $dt_string = overload::StrVal($date1);

        Carp::croak( "Cannot subtract $date2 from a $class object ($dt_string).\n"
                     . " Only a DateTime::Duration or DateTime object can "
                     . " be subtracted from a $class object." );
    }
}

sub add
{
    my $self = shift;

    return $self->add_duration( $self->duration_class->new(@_) );
}

sub subtract
{
    my $self = shift;

    return $self->subtract_duration( $self->duration_class->new(@_) );
}

sub subtract_duration { return $_[0]->add_duration( $_[1]->inverse ) }

{
    my @spec = ( { isa => 'DateTime::Duration' } );
    sub add_duration
    {
        my $self = shift;
        my ($dur) = validate_pos( @_, @spec );

        # simple optimization
        return $self if $dur->is_zero;

        my %deltas = $dur->deltas;

        # This bit isn't quite right since DateTime::Infinite::Future -
        # infinite duration should NaN
        foreach my $val ( values %deltas )
        {
            my $inf;
            if ( $val == INFINITY )
            {
                $inf = DateTime::Infinite::Future->new;
            }
            elsif ( $val == NEG_INFINITY )
            {
                $inf = DateTime::Infinite::Past->new;
            }

            if ($inf)
            {
                %$self = %$inf;
                bless $self, ref $inf;

                return $self;
            }
        }

        return $self if $self->is_infinite;

        if ( $deltas{days} )
        {
            $self->{local_rd_days} += $deltas{days};

            $self->{utc_year} += int( $deltas{days} / 365 ) + 1;
        }

        if ( $deltas{months} )
        {
            # For preserve mode, if it is the last day of the month, make
            # it the 0th day of the following month (which then will
            # normalize back to the last day of the new month).
            my ($y, $m, $d) = ( $dur->is_preserve_mode ?
                                $self->_rd2ymd( $self->{local_rd_days} + 1 ) :
                                $self->_rd2ymd( $self->{local_rd_days} )
                              );

            $d -= 1 if $dur->is_preserve_mode;

            if ( ! $dur->is_wrap_mode && $d > 28 )
            {
                # find the rd for the last day of our target month
                $self->{local_rd_days} = $self->_ymd2rd( $y, $m + $deltas{months} + 1, 0 );

                # what day of the month is it? (discard year and month)
                my $last_day = ($self->_rd2ymd( $self->{local_rd_days} ))[2];

                # if our original day was less than the last day,
                # use that instead
                $self->{local_rd_days} -= $last_day - $d if $last_day > $d;
            }
            else
            {
                $self->{local_rd_days} = $self->_ymd2rd( $y, $m + $deltas{months}, $d );
            }

            $self->{utc_year} += int( $deltas{months} / 12 ) + 1;
        }

        if ( $deltas{days} || $deltas{months} )
        {
            $self->_calc_utc_rd;

            $self->_handle_offset_modifier( $self->second );
        }

        if ( $deltas{minutes} )
        {
            $self->{utc_rd_secs} += $deltas{minutes} * 60;

            # This intentionally ignores leap seconds
            $self->_normalize_tai_seconds( $self->{utc_rd_days}, $self->{utc_rd_secs} );
        }

        if ( $deltas{seconds} || $deltas{nanoseconds} )
        {
            $self->{utc_rd_secs} += $deltas{seconds};

            if ( $deltas{nanoseconds} )
            {
                $self->{rd_nanosecs} += $deltas{nanoseconds};
                $self->_normalize_nanoseconds( $self->{utc_rd_secs}, $self->{rd_nanosecs} );
            }

            $self->_normalize_seconds;

            # This might be some big number much bigger than 60, but
            # that's ok (there are tests in 19leap_second.t to confirm
            # that)
            $self->_handle_offset_modifier( $self->second + $deltas{seconds} );
        }

        my $new =
            (ref $self)->from_object
                ( object => $self,
                  locale => $self->{locale},
                  ( $self->{formatter} ? ( formatter => $self->{formatter} ) : () ),
                 );

        %$self = %$new;

        return $self;
    }
}

sub _compare_overload
{
    # note: $_[1]->compare( $_[0] ) is an error when $_[1] is not a
    # DateTime (such as the INFINITY value)
    return $_[2] ? - $_[0]->compare( $_[1] ) : $_[0]->compare( $_[1] );
}

sub compare
{
    shift->_compare( @_, 0 );
}

sub compare_ignore_floating
{
    shift->_compare( @_, 1 );
}

sub _compare
{
    my ( $class, $dt1, $dt2, $consistent ) = ref $_[0] ? ( undef, @_ ) : @_;

    return undef unless defined $dt2;

    if ( ! ref $dt2 && ( $dt2 == INFINITY || $dt2 == NEG_INFINITY ) )
    {
        return $dt1->{utc_rd_days} <=> $dt2;
    }

    unless ( DateTime::Helpers::can( $dt1, 'utc_rd_values' )
             && DateTime::Helpers::can( $dt2, 'utc_rd_values' ) )
    {
        my $dt1_string = overload::StrVal($dt1);
        my $dt2_string = overload::StrVal($dt2);

        Carp::croak( "A DateTime object can only be compared to"
                     . " another DateTime object ($dt1_string, $dt2_string)." );
    }

    if ( ! $consistent &&
         DateTime::Helpers::can( $dt1, 'time_zone' ) &&
         DateTime::Helpers::can( $dt2, 'time_zone' )
       )
    {
        my $is_floating1 = $dt1->time_zone->is_floating;
        my $is_floating2 = $dt2->time_zone->is_floating;

        if ( $is_floating1 && ! $is_floating2 )
        {
            $dt1 = $dt1->clone->set_time_zone( $dt2->time_zone );
        }
        elsif ( $is_floating2 && ! $is_floating1 )
        {
            $dt2 = $dt2->clone->set_time_zone( $dt1->time_zone );
        }
    }

    my @dt1_components = $dt1->utc_rd_values;
    my @dt2_components = $dt2->utc_rd_values;

    foreach my $i ( 0..2 )
    {
        return $dt1_components[$i] <=> $dt2_components[$i]
            if $dt1_components[$i] != $dt2_components[$i]
    }

    return 0;
}

sub _string_equals_overload
{
    my ( $class, $dt1, $dt2 ) = ref $_[0] ? ( undef, @_ ) : @_;

    return unless
        (    DateTime::Helpers::can( $dt1, 'utc_rd_values' )
          && DateTime::Helpers::can( $dt2, 'utc_rd_values' )
        );

    $class ||= ref $dt1;
    return ! $class->compare( $dt1, $dt2 );
}

sub _string_not_equals_overload
{
    return ! _string_equals_overload(@_);
}

sub _normalize_nanoseconds
{
    use integer;

    # seconds, nanoseconds
    if ( $_[2] < 0 )
    {
        my $overflow = 1 + $_[2] / MAX_NANOSECONDS;
        $_[2] += $overflow * MAX_NANOSECONDS;
        $_[1] -= $overflow;
    }
    elsif ( $_[2] >= MAX_NANOSECONDS )
    {
        my $overflow = $_[2] / MAX_NANOSECONDS;
        $_[2] -= $overflow * MAX_NANOSECONDS;
        $_[1] += $overflow;
    }
}

# Many of the same parameters as new() but all of them are optional,
# and there are no defaults.
my $SetValidate =
    { map { my %copy = %{ $BasicValidate->{$_} };
            delete $copy{default};
            $copy{optional} = 1;
            $_ => \%copy }
      keys %$BasicValidate };

sub set
{
    my $self = shift;
    my %p = validate( @_, $SetValidate );

    my $new_dt = $self->_new_from_self(%p);

    %$self = %$new_dt;

    return $self;
}

sub set_year   { $_[0]->set( year => $_[1] ) }
sub set_month  { $_[0]->set( month => $_[1] ) }
sub set_day    { $_[0]->set( day => $_[1] ) }
sub set_hour   { $_[0]->set( hour => $_[1] ) }
sub set_minute { $_[0]->set( minute => $_[1] ) }
sub set_second { $_[0]->set( second => $_[1] ) }
sub set_nanosecond { $_[0]->set( nanosecond => $_[1] ) }

sub set_locale { $_[0]->set( locale => $_[1] ) }

sub set_formatter { $_[0]->{formatter} = $_[1] }

{
    my %TruncateDefault = ( month  => 1,
                            day    => 1,
                            hour   => 0,
                            minute => 0,
                            second => 0,
                            nanosecond => 0,
                          );
    my $re = join '|', 'year', 'week', grep { $_ ne 'nanosecond' } keys %TruncateDefault;
    my $spec = { to => { regex => qr/^(?:$re)/ } };

    sub truncate
    {
        my $self = shift;
        my %p = validate( @_, $spec );

        my %new;
        if ( $p{to} eq 'week' )
        {
            my $day_diff = $self->day_of_week - 1;

            if ($day_diff)
            {
                $self->add( days => -1 * $day_diff );
            }

            return $self->truncate( to => 'day' );
        }
        else
        {
            my $truncate;
            foreach my $f ( qw( year month day hour minute second nanosecond ) )
            {
                $new{$f} = $truncate ? $TruncateDefault{$f} : $self->$f();

                $truncate = 1 if $p{to} eq $f;
            }
        }

        my $new_dt = $self->_new_from_self(%new);

        %$self = %$new_dt;

        return $self;
    }
}

sub set_time_zone
{
    my ( $self, $tz ) = @_;

    # This is a bit of a hack but it works because time zone objects
    # are singletons, and if it doesn't work all we lose is a little
    # bit of speed.
    return $self if $self->{tz} eq $tz;

    my $was_floating = $self->{tz}->is_floating;

    $self->{tz} = ref $tz ? $tz : DateTime::TimeZone->new( name => $tz );

    $self->_handle_offset_modifier( $self->second, 1 );

    # if it either was or now is floating (but not both)
    if ( $self->{tz}->is_floating xor $was_floating )
    {
        $self->_calc_utc_rd;
    }
    elsif ( ! $was_floating )
    {
        $self->_calc_local_rd;
    }

    return $self;
}

sub STORABLE_freeze
{
    my $self = shift;
    my $cloning = shift;

    my $serialized = '';
    foreach my $key ( qw( utc_rd_days
                          utc_rd_secs
                          rd_nanosecs ) )
    {
        $serialized .= "$key:$self->{$key}|";
    }

    # not used yet, but may be handy in the future.
    $serialized .= "version:$VERSION";

    # Formatter needs to be returned as a reference since it may be
    # undef or a class name, and Storable will complain if extra
    # return values aren't refs
    return $serialized, $self->{locale}, $self->{tz}, \$self->{formatter};
}

sub STORABLE_thaw
{
    my $self = shift;
    my $cloning = shift;
    my $serialized = shift;

    my %serialized = map { split /:/ } split /\|/, $serialized;

    my ( $locale, $tz, $formatter );

    # more recent code version
    if (@_)
    {
        ( $locale, $tz, $formatter ) = @_;
    }
    else
    {
        $tz = DateTime::TimeZone->new( name => delete $serialized{tz} );

        $locale =
            DateTime::Locale->load( exists $serialized{language}
                                    ? delete $serialized{language}
                                    : delete $serialized{locale}
                                  );
    }

    delete $serialized{version};

    my $object = bless { utc_vals => [ $serialized{utc_rd_days},
                                       $serialized{utc_rd_secs},
                                       $serialized{rd_nanosecs},
                                     ],
                         tz       => $tz,
                       }, 'DateTime::_Thawed';

    my %formatter = defined $$formatter ? ( formatter => $$formatter ) : ();
    my $new = (ref $self)->from_object( object => $object,
                                        locale => $locale,
                                        %formatter,
                                      );

    %$self = %$new;

    return $self;
}


package DateTime::_Thawed;

sub utc_rd_values { @{ $_[0]->{utc_vals} } }

sub time_zone { $_[0]->{tz} }


1;

__END__

=head1 NAME

DateTime - A date and time object

=head1 SYNOPSIS

  use DateTime;

  $dt = DateTime->new( year   => 1964,
                       month  => 10,
                       day    => 16,
                       hour   => 16,
                       minute => 12,
                       second => 47,
                       nanosecond => 500000000,
                       time_zone => 'Asia/Taipei',
                     );

  $dt = DateTime->from_epoch( epoch => $epoch );
  $dt = DateTime->now; # same as ( epoch => time() )

  $year   = $dt->year;
  $month  = $dt->month;          # 1-12 - also mon

  $day    = $dt->day;            # 1-31 - also day_of_month, mday

  $dow    = $dt->day_of_week;    # 1-7 (Monday is 1) - also dow, wday

  $hour   = $dt->hour;           # 0-23
  $minute = $dt->minute;         # 0-59 - also min

  $second = $dt->second;         # 0-61 (leap seconds!) - also sec

  $doy    = $dt->day_of_year;    # 1-366 (leap years) - also doy

  $doq    = $dt->day_of_quarter; # 1.. - also doq

  $qtr    = $dt->quarter;        # 1-4

  # all of the start-at-1 methods above have correponding start-at-0
  # methods, such as $dt->day_of_month_0, $dt->month_0 and so on

  $ymd    = $dt->ymd;           # 2002-12-06
  $ymd    = $dt->ymd('/');      # 2002/12/06 - also date

  $mdy    = $dt->mdy;           # 12-06-2002
  $mdy    = $dt->mdy('/');      # 12/06/2002

  $dmy    = $dt->dmy;           # 06-12-2002
  $dmy    = $dt->dmy('/');      # 06/12/2002

  $hms    = $dt->hms;           # 14:02:29
  $hms    = $dt->hms('!');      # 14!02!29 - also time

  $is_leap  = $dt->is_leap_year;

  # these are localizable, see Locales section
  $month_name  = $dt->month_name; # January, February, ...
  $month_abbr  = $dt->month_abbr; # Jan, Feb, ...
  $day_name    = $dt->day_name;   # Monday, Tuesday, ...
  $day_abbr    = $dt->day_abbr;   # Mon, Tue, ...

  # May not work for all possible datetime, see the docs on this
  # method for more details.
  $epoch_time  = $dt->epoch;

  $dt2 = $dt + $duration_object;

  $dt3 = $dt - $duration_object;

  $duration_object = $dt - $dt2;

  $dt->set( year => 1882 );

  $dt->set_time_zone( 'America/Chicago' );

  $dt->set_formatter( $formatter );

=head1 DESCRIPTION

DateTime is a class for the representation of date/time combinations,
and is part of the Perl DateTime project.  For details on this project
please see L<http://datetime.perl.org/>.  The DateTime site has a FAQ
which may help answer many "how do I do X?" questions.  The FAQ is at
L<http://datetime.perl.org/?FAQ>.

It represents the Gregorian calendar, extended backwards in time
before its creation (in 1582).  This is sometimes known as the
"proleptic Gregorian calendar".  In this calendar, the first day of
the calendar (the epoch), is the first day of year 1, which
corresponds to the date which was (incorrectly) believed to be the
birth of Jesus Christ.

The calendar represented does have a year 0, and in that way differs
from how dates are often written using "BCE/CE" or "BC/AD".

For infinite datetimes, please see the
L<DateTime::Infinite|DateTime::Infinite> module.

=head1 USAGE

=head2 0-based Versus 1-based Numbers

The DateTime.pm module follows a simple consistent logic for
determining whether or not a given number is 0-based or 1-based.

Month, day of month, day of week, and day of year are 1-based.  Any
method that is 1-based also has an equivalent 0-based method ending in
"_0".  So for example, this class provides both C<day_of_week()> and
C<day_of_week_0()> methods.

The C<day_of_week_0()> method still treats Monday as the first day of
the week.

All I<time>-related numbers such as hour, minute, and second are
0-based.

Years are neither, as they can be both positive or negative, unlike
any other datetime component.  There I<is> a year 0.

There is no C<quarter_0()> method.

=head2 Error Handling

Some errors may cause this module to die with an error string.  This
can only happen when calling constructor methods, methods that change
the object, such as C<set()>, or methods that take parameters.
Methods that retrieve information about the object, such as C<year()>
or C<epoch()>, will never die.

=head2 Locales

All the object methods which return names or abbreviations return data
based on a locale.  This is done by setting the locale when
constructing a DateTime object.  There is also a C<DefaultLocale()>
class method which may be used to set the default locale for all
DateTime objects created.  If this is not set, then "en_US" is used.

=head2 Floating DateTimes

The default time zone for new DateTime objects, except where stated
otherwise, is the "floating" time zone.  This concept comes from the
iCal standard.  A floating datetime is one which is not anchored to
any particular time zone.  In addition, floating datetimes do not
include leap seconds, since we cannot apply them without knowing the
datetime's time zone.

The results of date math and comparison between a floating datetime
and one with a real time zone are not really valid, because one
includes leap seconds and the other does not.  Similarly, the results
of datetime math between two floating datetimes and two datetimes with
time zones are not really comparable.

If you are planning to use any objects with a real time zone, it is
strongly recommended that you B<do not> mix these with floating
datetimes.

=head2 Math

If you are going to be using doing date math, please read the section
L<How Datetime Math is Done>.

=head2 Time Zone Warnings

Determining the local time zone for a system can be slow. If C<$ENV{TZ}> is
not set, it may involve reading a number of files in F</etc> or elsewhere. If
you know that the local time zone won't change while your code is running, and
you need to make many objects for the local time zone, it is strongly
recommended that you retrieve the local time zone once and cache it:

  our $App::LocalTZ = DateTime::TimeZone->new( name => 'local' );

  ... # then everywhere else

  my $dt = DateTime->new( ..., time_zone => $App::LocalTZ );

DateTime itself does not do this internally because local time zones can
change, and there's no good way to determine if it's changed without doing all
the work to look it up.

Do not try to use named time zones (like "America/Chicago") with dates
very far in the future (thousands of years). The current
implementation of C<DateTime::TimeZone> will use a huge amount of
memory calculating all the DST changes from now until the future
date. Use UTC or the floating time zone and you will be safe.

=head2 Methods

=head3 Constructors

All constructors can die when invalid parameters are given.

=over 4

=item * DateTime->new( ... )

This class method accepts parameters for each date and time component:
"year", "month", "day", "hour", "minute", "second", "nanosecond".
It also accepts "locale", "time_zone", and "formatter" parameters.

  my $dt = DateTime->new( year   => 1066,
                          month  => 10,
                          day    => 25,
                          hour   => 7,
                          minute => 15,
                          second => 47,
                          nanosecond => 500000000,
                          time_zone  => 'America/Chicago',
                        );

DateTime validates the "month", "day", "hour", "minute", and "second",
and "nanosecond" parameters.  The valid values for these parameters are:

=over 8

=item * month

1-12

=item * day

1-31, and it must be within the valid range of days for the specified
month

=item * hour

0-23

=item * minute

0-59

=item * second

0-61 (to allow for leap seconds).  Values of 60 or 61 are only allowed
when they match actual leap seconds.

=item * nanosecond

>= 0

=back

=back

Invalid parameter types (like an array reference) will cause the
constructor to die.

The value for seconds may be from 0 to 61, to account for leap
seconds.  If you give a value greater than 59, DateTime does check to
see that it really matches a valid leap second.

All of the parameters are optional except for "year".  The "month" and
"day" parameters both default to 1, while the "hour", "minute",
"second", and "nanosecond" parameters all default to 0.

The "locale" parameter should be a string matching one of the valid
locales, or a C<DateTime::Locale> object.  See the
L<DateTime::Locale|DateTime::Locale> documentation for details.

The time_zone parameter can be either a scalar or a
C<DateTime::TimeZone> object.  A string will simply be passed to the
C<< DateTime::TimeZone->new >> method as its "name" parameter.  This
string may be an Olson DB time zone name ("America/Chicago"), an
offset string ("+0630"), or the words "floating" or "local".  See the
C<DateTime::TimeZone> documentation for more details.

The default time zone is "floating".

The "formatter" can be either a scalar or an object, but the class
specified by the scalar or the object must implement a
C<format_datetime()> method.

=head4 Parsing Dates

B<This module does not parse dates!> That means there is no
constructor to which you can pass things like "March 3, 1970 12:34".

Instead, take a look at the various C<DateTime::Format::*> modules on
CPAN. These parse all sorts of different date formats, and you're
bound to find something that can handle your particular needs.

=head4 Ambiguous Local Times

Because of Daylight Saving Time, it is possible to specify a local
time that is ambiguous.  For example, in the US in 2003, the
transition from to saving to standard time occurred on October 26, at
02:00:00 local time.  The local clock changed from 01:59:59 (saving
time) to 01:00:00 (standard time).  This means that the hour from
01:00:00 through 01:59:59 actually occurs twice, though the UTC time
continues to move forward.

If you specify an ambiguous time, then the latest UTC time is always
used, in effect always choosing standard time.  In this case, you can
simply subtract an hour to the object in order to move to saving time,
for example:

  # This object represent 01:30:00 standard time
  my $dt = DateTime->new( year   => 2003,
                          month  => 10,
                          day    => 26,
                          hour   => 1,
                          minute => 30,
                          second => 0,
                          time_zone => 'America/Chicago',
                        );

  print $dt->hms;  # prints 01:30:00

  # Now the object represent 01:30:00 saving time
  $dt->subtract( hours => 1 );

  print $dt->hms;  # still prints 01:30:00

Alternately, you could create the object with the UTC time zone, and
then call the C<set_time_zone()> method to change the time zone.  This
is a good way to ensure that the time is not ambiguous.

=head4 Invalid Local Times

Another problem introduced by Daylight Saving Time is that certain
local times just do not exist.  For example, in the US in 2003, the
transition from standard to saving time occurred on April 6, at the
change to 2:00:00 local time.  The local clock changes from 01:59:59
(standard time) to 03:00:00 (saving time).  This means that there is
no 02:00:00 through 02:59:59 on April 6!

Attempting to create an invalid time currently causes a fatal error.
This may change in future version of this module.

=over 4

=item * DateTime->from_epoch( epoch => $epoch, ... )

This class method can be used to construct a new DateTime object from
an epoch time instead of components.  Just as with the C<new()>
method, it accepts "time_zone", "locale", and "formatter" parameters.

If the epoch value is not an integer, the part after the decimal will
be converted to nanoseconds.  This is done in order to be compatible
with C<Time::HiRes>.  If the floating portion extends past 9 decimal
places, it will be truncated to nine, so that 1.1234567891 will become
1 second and 123,456,789 nanoseconds.

By default, the returned object will be in the UTC time zone.

=item * DateTime->now( ... )

This class method is equivalent to calling C<from_epoch()> with the
value returned from Perl's C<time()> function.  Just as with the
C<new()> method, it accepts "time_zone" and "locale" parameters.

By default, the returned object will be in the UTC time zone.

=item * DateTime->today( ... )

This class method is equivalent to:

  DateTime->now->truncate( to => 'day' );

=item * DateTime->from_object( object => $object, ... )

This class method can be used to construct a new DateTime object from
any object that implements the C<utc_rd_values()> method.  All
C<DateTime::Calendar> modules must implement this method in order to
provide cross-calendar compatibility.  This method accepts a
"locale" and "formatter" parameter

If the object passed to this method has a C<time_zone()> method, that
is used to set the time zone of the newly created C<DateTime.pm>
object.

Otherwise, the returned object will be in the floating time zone.

=item * DateTime->last_day_of_month( ... )

This constructor takes the same arguments as can be given to the
C<new()> method, except for "day".  Additionally, both "year" and
"month" are required.

=item * DateTime->from_day_of_year( ... )

This constructor takes the same arguments as can be given to the
C<new()> method, except that it does not accept a "month" or "day"
argument.  Instead, it requires both "year" and "day_of_year".  The
day of year must be between 1 and 366, and 366 is only allowed for
leap years.

=item * $dt->clone()

This object method returns a new object that is replica of the object
upon which the method is called.

=back

=head3 "Get" Methods

This class has many methods for retrieving information about an
object.

=over 4

=item * $dt->year()

Returns the year.

=item * $dt->ce_year()

Returns the year according to the BCE/CE numbering system.  The year
before year 1 in this system is year -1, aka "1 BCE".

=item * $dt->era_name()

Returns the long name of the current era, something like "Before
Christ".  See the L<Locales|/Locales> section for more details.

=item * $dt->era_abbr()

Returns the abbreviated name of the current era, something like "BC".
See the L<Locales|/Locales> section for more details.

=item * $dt->christian_era()

Returns a string, either "BC" or "AD", according to the year.

=item * $dt->secular_era()

Returns a string, either "BCE" or "CE", according to the year.

=item * $dt->year_with_era()

Returns a string containing the year immediately followed by its era
abbreviation.  The year is the absolute value of C<ce_year()>, so that
year 1 is "1AD" and year 0 is "1BC".

=item * $dt->year_with_christian_era()

Like C<year_with_era()>, but uses the christian_era() to get the era
name.

=item * $dt->year_with_secular_era()

Like C<year_with_era()>, but uses the secular_era() method to get the
era name.

=item * $dt->month()

=item * $dt->mon()

Returns the month of the year, from 1..12.

=item * $dt->month_name()

Returns the name of the current month.  See the
L<Locales|/Locales> section for more details.

=item * $dt->month_abbr()

Returns the abbreviated name of the current month.  See the
L<Locales|/Locales> section for more details.

=item * $dt->day_of_month()

=item * $dt->day()

=item * $dt->mday()

Returns the day of the month, from 1..31.

=item * $dt->day_of_week()

=item * $dt->wday()

=item * $dt->dow()

Returns the day of the week as a number, from 1..7, with 1 being
Monday and 7 being Sunday.

=item * $dt->local_day_of_week()

Returns the day of the week as a number, from 1..7. The day
corresponding to 1 will vary based on the locale.

=item * $dt->day_name()

Returns the name of the current day of the week.  See the
L<Locales|/Locales> section for more details.

=item * $dt->day_abbr()

Returns the abbreviated name of the current day of the week.  See the
L<Locales|/Locales> section for more details.

=item * $dt->day_of_year()

=item * $dt->doy()

Returns the day of the year.

=item * $dt->quarter()

Returns the quarter of the year, from 1..4.

=item * $dt->quarter_name()

Returns the name of the current quarter.  See the
L<Locales|/Locales> section for more details.

=item * $dt->quarter_abbr()

Returns the abbreviated name of the current quarter.  See the
L<Locales|/Locales> section for more details.

=item * $dt->day_of_quarter()

=item * $dt->doq()

Returns the day of the quarter.

=item * $dt->weekday_of_month()

Returns a number from 1..5 indicating which week day of the month this
is.  For example, June 9, 2003 is the second Monday of the month, and
so this method returns 2 for that day.

=item * $dt->ymd( $optional_separator ) - also $dt->date(...)

=item * $dt->mdy( $optional_separator )

=item * $dt->dmy( $optional_separator )

Each method returns the year, month, and day, in the order indicated
by the method name.  Years are zero-padded to four digits.  Months and
days are 0-padded to two digits.

By default, the values are separated by a dash (-), but this can be
overridden by passing a value to the method.

=item * $dt->hour()

Returns the hour of the day, from 0..23.

=item * $dt->hour_1()

Returns the hour of the day, from 1..24.

=item * $dt->hour_12()

Returns the hour of the day, from 1..12.

=item * $dt->hour_12_0()

Returns the hour of the day, from 0..11.

=item * $dt->am_or_pm()

Returns the appropriate localized abbreviation, depending on the
current hour.

=item * $dt->minute()

=item * $dt->min()

Returns the minute of the hour, from 0..59.

=item * $dt->second()

=item * $dt->sec()

Returns the second, from 0..61.  The values 60 and 61 are used for
leap seconds.

=item * $dt->fractional_second()

Returns the second, as a real number from 0.0 until 61.999999999

The values 60 and 61 are used for leap seconds.

=item * $dt->millisecond()

Returns the fractional part of the second as milliseconds (1E-3 seconds).

Half a second is 500 milliseconds.

=item * $dt->microsecond()

Returns the fractional part of the second as microseconds (1E-6
seconds).  This value will be rounded to an integer.

Half a second is 500_000 microseconds.  This value will be rounded to
an integer.

=item * $dt->nanosecond()

Returns the fractional part of the second as nanoseconds (1E-9 seconds).

Half a second is 500_000_000 nanoseconds.

=item * $dt->hms( $optional_separator )

=item * $dt->time( $optional_separator )

Returns the hour, minute, and second, all zero-padded to two digits.
If no separator is specified, a colon (:) is used by default.

=item * $dt->datetime()

=item * $dt->iso8601()

This method is equivalent to:

  $dt->ymd('-') . 'T' . $dt->hms(':')

=item * $dt->is_leap_year()

This method returns a true or false indicating whether or not the
datetime object is in a leap year.

=item * $dt->week()

 ($week_year, $week_number) = $dt->week;

Returns information about the calendar week which contains this
datetime object. The values returned by this method are also available
separately through the week_year and week_number methods.

The first week of the year is defined by ISO as the one which contains
the fourth day of January, which is equivalent to saying that it's the
first week to overlap the new year by at least four days.

Typically the week year will be the same as the year that the object
is in, but dates at the very beginning of a calendar year often end up
in the last week of the prior year, and similarly, the final few days
of the year may be placed in the first week of the next year.

=item * $dt->week_year()

Returns the year of the week.

=item * $dt->week_number()

Returns the week of the year, from 1..53.

=item * $dt->week_of_month()

The week of the month, from 0..5.  The first week of the month is the
first week that contains a Thursday.  This is based on the ICU
definition of week of month, and correlates to the ISO8601 week of
year definition.  A day in the week I<before> the week with the first
Thursday will be week 0.

=item * $dt->jd()

=item * $dt->mjd()

These return the Julian Day and Modified Julian Day, respectively.
The value returned is a floating point number.  The fractional portion
of the number represents the time portion of the datetime.

=item * $dt->time_zone()

This returns the C<DateTime::TimeZone> object for the datetime object.

=item * $dt->offset()

This returns the offset from UTC, in seconds, of the datetime object
according to the time zone.

=item * $dt->is_dst()

Returns a boolean indicating whether or not the datetime object is
currently in Daylight Saving Time or not.

=item * $dt->time_zone_long_name()

This is a shortcut for C<< $dt->time_zone->name >>.  It's provided so
that one can use "%{time_zone_long_name}" as a strftime format
specifier.

=item * $dt->time_zone_short_name()

This method returns the time zone abbreviation for the current time
zone, such as "PST" or "GMT".  These names are B<not> definitive, and
should not be used in any application intended for general use by
users around the world.

=item * $dt->strftime( $format, ... )

This method implements functionality similar to the C<strftime()>
method in C.  However, if given multiple format strings, then it will
return multiple scalars, one for each format string.

See the L<strftime Patterns> section for a list of all possible
strftime patterns.

If you give a pattern that doesn't exist, then it is simply treated as
text.

=item * $dt->format_cldr( $format, ... )

This method implements formatting based on the CLDR date patterns.  If
given multiple format strings, then it will return multiple scalars,
one for each format string.

See the L<CLDR Patterns> section for a list of all possible CLDR
patterns.

If you give a pattern that doesn't exist, then it is simply treated as
text.

=item * $dt->epoch()

Return the UTC epoch value for the datetime object.  Internally, this
is implemented using C<Time::Local>, which uses the Unix epoch even on
machines with a different epoch (such as MacOS).  Datetimes before the
start of the epoch will be returned as a negative number.

The return value from this method is always an integer.

Since the epoch does not account for leap seconds, the epoch time for
1972-12-31T23:59:60 (UTC) is exactly the same as that for
1973-01-01T00:00:00.

This module uses C<Time::Local> to calculate the epoch, which may or
may not handle epochs before 1904 or after 2038 (depending on the size
of your system's integers, and whether or not Perl was compiled with
64-bit int support).

=item * $dt->hires_epoch()

Returns the epoch as a floating point number.  The floating point
portion of the value represents the nanosecond value of the object.
This method is provided for compatibility with the C<Time::HiRes>
module.

=item * $dt->is_finite()

=item * $dt->is_infinite

These methods allow you to distinguish normal datetime objects from
infinite ones.  Infinite datetime objects are documented in
L<DateTime::Infinite|DateTime::Infinite>.

=item * $dt->utc_rd_values()

Returns the current UTC Rata Die days, seconds, and nanoseconds as a
three element list.  This exists primarily to allow other calendar
modules to create objects based on the values provided by this object.

=item * $dt->local_rd_values()

Returns the current local Rata Die days, seconds, and nanoseconds as a
three element list.  This exists for the benefit of other modules
which might want to use this information for date math, such as
C<DateTime::Event::Recurrence>.

=item * $dt->leap_seconds()

Returns the number of leap seconds that have happened up to the
datetime represented by the object.  For floating datetimes, this
always returns 0.

=item * $dt->utc_rd_as_seconds()

Returns the current UTC Rata Die days and seconds purely as seconds.
This number ignores any fractional seconds stored in the object,
as well as leap seconds.

=item * $dt->local_rd_as_seconds() - deprecated

Returns the current local Rata Die days and seconds purely as seconds.
This number ignores any fractional seconds stored in the object,
as well as leap seconds.

=item * $dt->locale()

Returns the current locale object.

=item * $dt->formatter()

Returns current formatter object or class. See L<Formatters And
Stringification> for details.

=back

=head3 "Set" Methods

The remaining methods provided by C<DateTime.pm>, except where otherwise
specified, return the object itself, thus making method chaining
possible. For example:

  my $dt = DateTime->now->set_time_zone( 'Australia/Sydney' );

  my $first = DateTime
                ->last_day_of_month( year => 2003, month => 3 )
                ->add( days => 1 )
                ->subtract( seconds => 1 );

=over 4

=item * $dt->set( .. )

This method can be used to change the local components of a date time,
or its locale.  This method accepts any parameter allowed by the
C<new()> method except for "time_zone".  Time zones may be set using
the C<set_time_zone()> method.

This method performs parameters validation just as is done in the
C<new()> method.

=item * $dt->set_year()

=item * $dt->set_month()

=item * $dt->set_day()

=item * $dt->set_hour()

=item * $dt->set_minute()

=item * $dt->set_second()

=item * $dt->set_nanosecond()

=item * $dt->set_locale()

These are shortcuts to calling C<set()> with a single key.  They all
take a single parameter.

=item * $dt->truncate( to => ... )

This method allows you to reset some of the local time components in
the object to their "zero" values.  The "to" parameter is used to
specify which values to truncate, and it may be one of "year",
"month", "week", "day", "hour", "minute", or "second".  For example,
if "month" is specified, then the local day becomes 1, and the hour,
minute, and second all become 0.

If "week" is given, then the datetime is set to the beginning of the
week in which it occurs, and the time components are all set to 0.

=item * $dt->set_time_zone( $tz )

This method accepts either a time zone object or a string that can be
passed as the "name" parameter to C<< DateTime::TimeZone->new() >>.
If the new time zone's offset is different from the old time zone,
then the I<local> time is adjusted accordingly.

For example:

  my $dt = DateTime->new( year => 2000, month => 5, day => 10,
                          hour => 15, minute => 15,
                          time_zone => 'America/Los_Angeles', );

  print $dt->hour; # prints 15

  $dt->set_time_zone( 'America/Chicago' );

  print $dt->hour; # prints 17

If the old time zone was a floating time zone, then no adjustments to
the local time are made, except to account for leap seconds.  If the
new time zone is floating, then the I<UTC> time is adjusted in order
to leave the local time untouched.

Fans of Tsai Ming-Liang's films will be happy to know that this does
work:

  my $dt = DateTime->now( time_zone => 'Asia/Taipei' );

  $dt->set_time_zone( 'Europe/Paris' );

Yes, now we can know "ni3 na4 bian1 ji2dian3?"

=item * $dt->set_formatter( $formatter )

Set the formatter for the object. See L<Formatters And
Stringification> for details.

=back

=head3 Math Methods

Like the set methods, math related methods always return the object
itself, to allow for chaining:

  $dt->add( days => 1 )->subtract( seconds => 1 );

=over 4

=item * $dt->duration_class()

This returns C<DateTime::Duration>, but exists so that a subclass of
C<DateTime.pm> can provide a different value.

=item * $dt->add_duration( $duration_object )

This method adds a C<DateTime::Duration> to the current datetime.  See
the L<DateTime::Duration|DateTime::Duration> docs for more details.

=item * $dt->add( DateTime::Duration->new parameters )

This method is syntactic sugar around the C<add_duration()> method.  It
simply creates a new C<DateTime::Duration> object using the parameters
given, and then calls the C<add_duration()> method.

=item * $dt->subtract_duration( $duration_object )

When given a C<DateTime::Duration> object, this method simply calls
C<invert()> on that object and passes that new duration to the
C<add_duration> method.

=item * $dt->subtract( DateTime::Duration->new parameters )

Like C<add()>, this is syntactic sugar for the C<subtract_duration()>
method.

=item * $dt->subtract_datetime( $datetime )

This method returns a new C<DateTime::Duration> object representing
the difference between the two dates.  The duration is B<relative> to
the object from which C<$datetime> is subtracted.  For example:

    2003-03-15 00:00:00.00000000
 -  2003-02-15 00:00:00.00000000

 -------------------------------

 = 1 month

Note that this duration is not an absolute measure of the amount of
time between the two datetimes, because the length of a month varies,
as well as due to the presence of leap seconds.

The returned duration may have deltas for months, days, minutes,
seconds, and nanoseconds.

=item * $dt->delta_md( $datetime )

=item * $dt->delta_days( $datetime )

Each of these methods returns a new C<DateTime::Duration> object
representing some portion of the difference between two datetimes.
The C<delta_md()> method returns a duration which contains only the
month and day portions of the duration is represented.  The
C<delta_days()> method returns a duration which contains only days.

The C<delta_md> and C<delta_days> methods truncate the duration so
that any fractional portion of a day is ignored.  Both of these
methods operate on the date portion of a datetime only, and so
effectively ignore the time zone.

Unlike the subtraction methods, B<these methods always return a
positive (or zero) duration>.

=item * $dt->delta_ms( $datetime )

Returns a duration which contains only minutes and seconds.  Any day
and month differences to minutes are converted to minutes and
seconds. This method also B<always return a positive (or zero)
duration>.

=item * $dt->subtract_datetime_absolute( $datetime )

This method returns a new C<DateTime::Duration> object representing
the difference between the two dates in seconds and nanoseconds.  This
is the only way to accurately measure the absolute amount of time
between two datetimes, since units larger than a second do not
represent a fixed number of seconds.

=back

=head3 Class Methods

=over 4

=item * DateTime->DefaultLocale( $locale )

This can be used to specify the default locale to be used when
creating DateTime objects.  If unset, then "en_US" is used.

=item * DateTime->compare( $dt1, $dt2 )

=item * DateTime->compare_ignore_floating( $dt1, $dt2 )

  $cmp = DateTime->compare( $dt1, $dt2 );

  $cmp = DateTime->compare_ignore_floating( $dt1, $dt2 );

Compare two DateTime objects.  The semantics are compatible with Perl's
C<sort()> function; it returns -1 if $dt1 < $dt2, 0 if $dt1 == $dt2, 1 if $dt1
> $dt2.

If one of the two DateTime objects has a floating time zone, it will
first be converted to the time zone of the other object.  This is what
you want most of the time, but it can lead to inconsistent results
when you compare a number of DateTime objects, some of which are
floating, and some of which are in other time zones.

If you want to have consistent results (because you want to sort a
number of objects, for example), you can use the
C<compare_ignore_floating()> method:

  @dates = sort { DateTime->compare_ignore_floating($a, $b) } @dates;

In this case, objects with a floating time zone will be sorted as if
they were UTC times.

Since DateTime objects overload comparison operators, this:

  @dates = sort @dates;

is equivalent to this:

  @dates = sort { DateTime->compare($a, $b) } @dates;

DateTime objects can be compared to any other calendar class that
implements the C<utc_rd_values()> method.

=back

=head2 How Datetime Math is Done

It's important to have some understanding of how datetime math is
implemented in order to effectively use this module and
C<DateTime::Duration>.

=head3 Making Things Simple

If you want to simplify your life and not have to think too hard about
the nitty-gritty of datetime math, I have several recommendations:

=over 4

=item * use the floating time zone

If you do not care about time zones or leap seconds, use the
"floating" timezone:

  my $dt = DateTime->now( time_zone => 'floating' );

Math done on two objects in the floating time zone produces very
predictable results.

=item * use UTC for all calculations

If you do care about time zones (particularly DST) or leap seconds,
try to use non-UTC time zones for presentation and user input only.
Convert to UTC immediately and convert back to the local time zone for
presentation:

  my $dt = DateTime->new( %user_input, time_zone => $user_tz );
  $dt->set_time_zone('UTC');

  # do various operations - store it, retrieve it, add, subtract, etc.

  $dt->set_time_zone($user_tz);
  print $dt->datetime;

=item * math on non-UTC time zones

If you need to do date math on objects with non-UTC time zones, please
read the caveats below carefully.  The results C<DateTime.pm> produces are
predictable and correct, and mostly intuitive, but datetime math gets
very ugly when time zones are involved, and there are a few strange
corner cases involving subtraction of two datetimes across a DST
change.

If you can always use the floating or UTC time zones, you can skip
ahead to L<Leap Seconds and Date Math|Leap Seconds and Date Math>

=item * date vs datetime math

If you only care about the date (calendar) portion of a datetime, you
should use either C<delta_md()> or C<delta_days()>, not
C<subtract_datetime()>.  This will give predictable, unsurprising
results, free from DST-related complications.

=item * subtract_datetime() and add_duration()

You must convert your datetime objects to the UTC time zone before
doing date math if you want to make sure that the following formulas
are always true:

  $dt2 - $dt1 = $dur
  $dt1 + $dur = $dt2
  $dt2 - $dur = $dt1

Note that using C<delta_days> ensures that this formula always works,
regardless of the timezone of the objects involved, as does using
C<subtract_datetime_absolute()>. Other methods of subtraction are not
always reversible.

=back

=head3 Adding a Duration to a Datetime

The parts of a duration can be broken down into five parts.  These are
months, days, minutes, seconds, and nanoseconds.  Adding one month to
a date is different than adding 4 weeks or 28, 29, 30, or 31 days.
Similarly, due to DST and leap seconds, adding a day can be different
than adding 86,400 seconds, and adding a minute is not exactly the
same as 60 seconds.

We cannot convert between these units, except for seconds and
nanoseconds, because there is no fixed conversion between the two
units, because of things like leap seconds, DST changes, etc.

C<DateTime.pm> always adds (or subtracts) days, then months, minutes, and then
seconds and nanoseconds.  If there are any boundary overflows, these are
normalized at each step.  For the days and months the local (not UTC) values
are used.  For minutes and seconds, the local values are used.  This generally
just works.

This means that adding one month and one day to February 28, 2003 will
produce the date April 1, 2003, not March 29, 2003.

  my $dt = DateTime->new( year => 2003, month => 2, day => 28 );

  $dt->add( months => 1, days => 1 );

  # 2003-04-01 - the result

On the other hand, if we add months first, and then separately add
days, we end up with March 29, 2003:

  $dt->add( months => 1 )->add( days => 1 );

  # 2003-03-29

We see similar strangeness when math crosses a DST boundary:

  my $dt = DateTime->new( year => 2003, month => 4, day => 5,
                          hour => 1, minute => 58,
                          time_zone => "America/Chicago",
                        );

  $dt->add( days => 1, minutes => 3 );
  # 2003-04-06 02:01:00

  $dt->add( minutes => 3 )->add( days => 1 );
  # 2003-04-06 03:01:00

Note that if you converted the datetime object to UTC first you would
get predictable results.

If you want to know how many seconds a duration object represents, you
have to add it to a datetime to find out, so you could do:

 my $now = DateTime->now( time_zone => 'UTC' );
 my $later = $now->clone->add_duration($duration);

 my $seconds_dur = $later->subtract_datetime_absolute($now);

This returns a duration which only contains seconds and nanoseconds.

If we were add the duration to a different datetime object we might
get a different number of seconds.

If you need to do lots of work with durations, take a look at Rick
Measham's C<DateTime::Format::Duration> module, which lets you present
information from durations in many useful ways.

There are other subtract/delta methods in DateTime.pm to generate
different types of durations.  These methods are
C<subtract_datetime()>, C<subtract_datetime_absolute()>,
C<delta_md()>, C<delta_days()>, and C<delta_ms()>.

=head3 Datetime Subtraction

Date subtraction is done solely based on the two object's local
datetimes, with one exception to handle DST changes.  Also, if the two
datetime objects are in different time zones, one of them is converted
to the other's time zone first before subtraction.  This is best
explained through examples:

The first of these probably makes the most sense:

    my $dt1 = DateTime->new( year => 2003, month => 5, day => 6,
                             time_zone => 'America/Chicago',
                           );
    # not DST

    my $dt2 = DateTime->new( year => 2003, month => 11, day => 6,
                             time_zone => 'America/Chicago',
                           );
    # is DST

    my $dur = $dt2->subtract_datetime($dt1);
    # 6 months

Nice and simple.

This one is a little trickier, but still fairly logical:

    my $dt1 = DateTime->new( year => 2003, month => 4, day => 5,
                             hour => 1, minute => 58,
                             time_zone => "America/Chicago",
                           );
    # is DST

    my $dt2 = DateTime->new( year => 2003, month => 4, day => 7,
                             hour => 2, minute => 1,
                             time_zone => "America/Chicago",
                           );
    # not DST

    my $dur = $dt2->subtract_datetime($dt1);
    # 2 days and 3 minutes

Which contradicts the result this one gives, even though they both
make sense:

    my $dt1 = DateTime->new( year => 2003, month => 4, day => 5,
                             hour => 1, minute => 58,
                             time_zone => "America/Chicago",
                           );
    # is DST

    my $dt2 = DateTime->new( year => 2003, month => 4, day => 6,
                             hour => 3, minute => 1,
                             time_zone => "America/Chicago",
                           );
    # not DST

    my $dur = $dt2->subtract_datetime($dt1);
    # 1 day and 3 minutes

This last example illustrates the "DST" exception mentioned earlier.
The exception accounts for the fact 2003-04-06 only lasts 23 hours.

And finally:

    my $dt2 = DateTime->new( year => 2003, month => 10, day => 26,
                             hour => 1,
                             time_zone => 'America/Chicago',
                           );

    my $dt1 = $dt2->clone->subtract( hours => 1 );

    my $dur = $dt2->subtract_datetime($dt1);
    # 60 minutes

This seems obvious until you realize that subtracting 60 minutes from
C<$dt2> in the above example still leaves the clock time at
"01:00:00".  This time we are accounting for a 25 hour day.

=head3 Reversibility

Date math operations are not always reversible.  This is because of
the way that addition operations are ordered.  As was discussed
earlier, adding 1 day and 3 minutes in one call to C<add()> is not the
same as first adding 3 minutes and 1 day in two separate calls.

If we take a duration returned from C<subtract_datetime()> and then
try to add or subtract that duration from one of the datetimes we just
used, we sometimes get interesting results:

  my $dt1 = DateTime->new( year => 2003, month => 4, day => 5,
                           hour => 1, minute => 58,
                           time_zone => "America/Chicago",
                         );

  my $dt2 = DateTime->new( year => 2003, month => 4, day => 6,
                           hour => 3, minute => 1,
                           time_zone => "America/Chicago",
                         );

  my $dur = $dt2->subtract_datetime($dt1);
  # 1 day and 3 minutes

  $dt1->add_duration($dur);
  # gives us $dt2

  $dt2->subtract_duration($dur);
  # gives us 2003-04-05 02:58:00 - 1 hour later than $dt1

The C<subtract_dauration()> operation gives us a (perhaps) unexpected
answer because it first subtracts one day to get 2003-04-05T03:01:00
and then subtracts 3 minutes to get the final result.

If we explicitly reverse the order we can get the original value of
C<$dt1>. This can be facilitated by C<DateTime::Duration>'s
C<calendar_duration()> and C<clock_duration()> methods:

  $dt2->subtract_duration( $dur->clock_duration )
      ->subtract_duration( $dur->calendar_duration );

=head3 Leap Seconds and Date Math

The presence of leap seconds can cause even more anomalies in date
math.  For example, the following is a legal datetime:

  my $dt = DateTime->new( year => 1972, month => 12, day => 31,
                          hour => 23, minute => 59, second => 60,
                          time_zone => 'UTC' );

If we do the following:

 $dt->add( months => 1 );

Then the datetime is now "1973-02-01 00:00:00", because there is no
23:59:60 on 1973-01-31.

Leap seconds also force us to distinguish between minutes and seconds
during date math.  Given the following datetime:

  my $dt = DateTime->new( year => 1972, month => 12, day => 31,
                          hour => 23, minute => 59, second => 30,
                          time_zone => 'UTC' );

we will get different results when adding 1 minute than we get if we
add 60 seconds.  This is because in this case, the last minute of the
day, beginning at 23:59:00, actually contains 61 seconds.

Here are the results we get:

  # 1972-12-31 23:59:30 - our starting datetime

  $dt->clone->add( minutes => 1 );
  # 1973-01-01 00:00:30 - one minute later

  $dt->clone->add( seconds => 60 );
  # 1973-01-01 00:00:29 - 60 seconds later

  $dt->clone->add( seconds => 61 );
  # 1973-01-01 00:00:30 - 61 seconds later

=head3 Local vs. UTC and 24 hours vs. 1 day

When math crosses a daylight saving boundary, a single day may have
more or less than 24 hours.

For example, if you do this:

  my $dt = DateTime->new( year => 2003, month => 4, day => 5,
                          hour => 2,
                          time_zone => 'America/Chicago',
                        );
  $dt->add( days => 1 );

then you will produce an I<invalid> local time, and therefore an
exception will be thrown.

However, this works:

  my $dt = DateTime->new( year => 2003, month => 4, day => 5,
                          hour => 2,
                          time_zone => 'America/Chicago',
                        );
  $dt->add( hours => 24 );

and produces a datetime with the local time of "03:00".

If all this makes your head hurt, there is a simple alternative.  Just
convert your datetime object to the "UTC" time zone before doing date
math on it, and switch it back to the local time zone afterwards.
This avoids the possibility of having date math throw an exception,
and makes sure that 1 day equals 24 hours.  Of course, this may not
always be desirable, so caveat user!

=head2 Overloading

This module explicitly overloads the addition (+), subtraction (-),
string and numeric comparison operators.  This means that the
following all do sensible things:

  my $new_dt = $dt + $duration_obj;

  my $new_dt = $dt - $duration_obj;

  my $duration_obj = $dt - $new_dt;

  foreach my $dt ( sort @dts ) { ... }

Additionally, the fallback parameter is set to true, so other
derivable operators (+=, -=, etc.) will work properly.  Do not expect
increment (++) or decrement (--) to do anything useful.

If you attempt to sort DateTime objects with non-DateTime.pm objects
or scalars (strings, number, whatever) then an exception will be
thrown. Using the string comparison operators, C<eq> or C<ne>, to
compare a DateTime.pm always returns false.

The module also overloads stringification to use the C<iso8601()>
method.

=head2 Formatters And Stringification

You can optionally specify a "formatter", which is usually a
DateTime::Format::* object/class, to control the stringification of
the DateTime object.

Any of the constructor methods can accept a formatter argument:

  my $formatter = DateTime::Format::Strptime->new(...);
  my $dt = DateTime->new(year => 2004, formatter => $formatter);

Or, you can set it afterwards:

  $dt->set_formatter($formatter);
  $formatter = $dt->formatter();

Once you set the formatter, the overloaded stringification method will
use the formatter. If unspecified, the C<iso8601()> method is used.

A formatter can be handy when you know that in your application you
want to stringify your DateTime objects into a special format all the
time, for example to a different language.

If you provide a formatter class name or object, it must implement a
C<format_datetime> method. This method will be called with just the
DateTime object as its argument.

=head2 strftime Patterns

The following patterns are allowed in the format string given to the
C<< $dt->strftime() >> method:

=over 4

=item * %a

The abbreviated weekday name.

=item * %A

The full weekday name.

=item * %b

The abbreviated month name.

=item * %B

The full month name.

=item * %c

The default datetime format for the object's locale.

=item * %C

The century number (year/100) as a 2-digit integer.

=item * %d

The day of the month as a decimal number (range 01 to 31).

=item * %D

Equivalent to %m/%d/%y.  This is not a good standard format if you
want folks from both the United States and the rest of the world to
understand the date!

=item * %e

Like %d, the day of the month as a decimal number, but a leading zero
is replaced by a space.

=item * %F

Equivalent to %Y-%m-%d (the ISO 8601 date format)

=item * %G

The ISO 8601 year with century as a decimal number.  The 4-digit year
corresponding to the ISO week number (see %V).  This has the same
format and value as %Y, except that if the ISO week number belongs to
the previous or next year, that year is used instead. (TZ)

=item * %g

Like %G, but without century, i.e., with a 2-digit year (00-99).

=item * %h

Equivalent to %b.

=item * %H

The hour as a decimal number using a 24-hour clock (range 00 to 23).

=item * %I

The hour as a decimal number using a 12-hour clock (range 01 to 12).

=item * %j

The day of the year as a decimal number (range 001 to 366).

=item * %k

The hour (24-hour clock) as a decimal number (range 0 to 23); single
digits are preceded by a blank. (See also %H.)

=item * %l

The hour (12-hour clock) as a decimal number (range 1 to 12); single
digits are preceded by a blank. (See also %I.)

=item * %m

The month as a decimal number (range 01 to 12).

=item * %M

The minute as a decimal number (range 00 to 59).

=item * %n

A newline character.

=item * %N

The fractional seconds digits. Default is 9 digits (nanoseconds).

  %3N   milliseconds (3 digits)
  %6N   microseconds (6 digits)
  %9N   nanoseconds  (9 digits)

=item * %p

Either `AM' or `PM' according to the given time value, or the
corresponding strings for the current locale.  Noon is treated as `pm'
and midnight as `am'.

=item * %P

Like %p but in lowercase: `am' or `pm' or a corresponding string for
the current locale.

=item * %r

The time in a.m.  or p.m. notation.  In the POSIX locale this is
equivalent to `%I:%M:%S %p'.

=item * %R

The time in 24-hour notation (%H:%M). (SU) For a version including the
seconds, see %T below.

=item * %s

The number of seconds since the epoch.

=item * %S

The second as a decimal number (range 00 to 61).

=item * %t

A tab character.

=item * %T

The time in 24-hour notation (%H:%M:%S).

=item * %u

The day of the week as a decimal, range 1 to 7, Monday being 1.  See
also %w.

=item * %U

The week number of the current year as a decimal number, range 00 to
53, starting with the first Sunday as the first day of week 01. See
also %V and %W.

=item * %V

The ISO 8601:1988 week number of the current year as a decimal number,
range 01 to 53, where week 1 is the first week that has at least 4
days in the current year, and with Monday as the first day of the
week. See also %U and %W.

=item * %w

The day of the week as a decimal, range 0 to 6, Sunday being 0.  See
also %u.

=item * %W

The week number of the current year as a decimal number, range 00 to
53, starting with the first Monday as the first day of week 01.

=item * %x

The default date format for the object's locale.

=item * %X

The default time format for the object's locale.

=item * %y

The year as a decimal number without a century (range 00 to 99).

=item * %Y

The year as a decimal number including the century.

=item * %z

The time-zone as hour offset from UTC.  Required to emit
RFC822-conformant dates (using "%a, %d %b %Y %H:%M:%S %z").

=item * %Z

The time zone or name or abbreviation.

=item * %%

A literal `%' character.

=item * %{method}

Any method name may be specified using the format C<%{method}> name
where "method" is a valid C<DateTime.pm> object method.

=back

=head2 CLDR Patterns

The CLDR pattern language is both more powerful and more complex than
strftime. Unlike strftime patterns, you often have to explicitly
escape text that you do not want formatted, as the patterns are simply
letters without any prefix.

For example, "yyyy-MM-dd" is a valid CLDR pattern. If you want to
include any lower or upper case ASCII characters as-is, you can
surround them with single quotes ('). If you want to include a single
quote, you must escape it as two single quotes ('').

  'Today is ' EEEE
  'It is now' h 'o''clock' a

Spaces and any non-letter text will always be passed through as-is.

Many CLDR patterns which produce numbers will pad the number with
leading zeroes depending on the length of the format specifier. For
example, "h" represents the current hour from 1-12. If you specify
"hh" then the 1-9 will have a leading zero prepended.

However, CLDR often uses five of a letter to represent the narrow form
of a pattern. This inconsistency is necessary for backwards
compatibility.

CLDR often distinguishes between the "format" and "stand-alone" forms
of a pattern. The format pattern is used when the thing in question is
being placed into a larger string. The stand-alone form is used when
displaying that item by itself, for example in a calendar.

It also often provides three sizes for each item, wide (the full
name), abbreviated, and narrow. The narrow form is often just a single
character, for example "T" for "Tuesday", and may not be unique.

CLDR provides a fairly complex system for localizing time zones that
we ignore entirely. The time zone patterns just use the information
provided by C<DateTime::TimeZone>, and I<do not follow the CLDR spec>.

The output of a CLDR pattern is always localized, when applicable.

CLDR provides the following patterns:

=over 4

=item * G{1,3}

The abbreviated era (BC, AD).

=item * GGGG

The wide era (Before Christ, Anno Domini).

=item * GGGGG

The narrow era, if it exists (and it mostly doesn't).

=item * y and y{3,}

The year, zero-prefixed as needed. Negative years will start with a "-",
and this will be included in the length calculation.

In other, words the "yyyyy" pattern will format year -1234 as "-1234", not
"-01234".

=item * yy

This is a special case. It always produces a two-digit year, so "1976" becomes
"76". Negative years will start with a "-", making them one character longer.

=item * Y{1,}

The week of the year, from C<< $dt->week_year() >>.

=item * u{1,}

Same as "y" except that "uu" is not a special case.

=item * Q{1,2}

The quarter as a number (1..4).

=item * QQQ

The abbreviated format form for the quarter.

=item * QQQQ

The wide format form for the quarter.

=item * q{1,2}

The quarter as a number (1..4).

=item * qqq

The abbreviated stand-alone form for the quarter.

=item * qqqq

The wide stand-alone form for the quarter.

=item * M{1,2]

The numerical month.

=item * MMM

The abbreviated format form for the month.

=item * MMMM

The wide format form for the month.

=item * MMMMM

The narrow format form for the month.

=item * L{1,2]

The numerical month.

=item * LLL

The abbreviated stand-alone form for the month.

=item * LLLL

The wide stand-alone form for the month.

=item * LLLLL

The narrow stand-alone form for the month.

=item * w{1,2}

The week of the year, from C<< $dt->week_number() >>.

=item * W

The week of the month, from C<< $dt->week_of_month() >>.

=item * d{1,2}

The numeric day of of the month.

=item * D{1,3}

The numeric day of of the year.

=item * F

The day of the week in the month, from C<< $dt->weekday_of_month() >>.

=item * g{1,}

The modified Julian day, from C<< $dt->mjd() >>.

=item * E{1,3} and eee

The abbreviated format form for the day of the week.

=item * EEEE and eeee

The wide format form for the day of the week.

=item * EEEEE and eeeee

The narrow format form for the day of the week.

=item * e{1,2}

The I<local> numeric day of the week, from 1 to 7. This number depends
on what day is considered the first day of the week, which varies by
locale. For example, in the US, Sunday is the first day of the week,
so this returns 2 for Monday.

=item * c

The numeric day of the week from 1 to 7, treating Monday as the first
of the week, regardless of locale.

=item * ccc

The abbreviated stand-alone form for the day of the week.

=item * cccc

The wide stand-alone form for the day of the week.

=item * ccccc

The narrow format form for the day of the week.

=item * a

The localized form of AM or PM for the time.

=item * h{1,2}

The hour from 1-12.

=item * H{1,2}

The hour from 0-23.

=item * K{1,2}

The hour from 0-11.

=item * k{1,2}

The hour from 1-24.

=item * j{1,2}

The hour, in 12 or 24 hour form, based on the preferred form for the
locale. In other words, this is equivalent to either "h{1,2}" or
"H{1,2}".

=item * m{1,2}

The minute.

=item * s{1,2}

The second.

=item * S{1,}

The fractional portion of the seconds, rounded based on the length of
the specifier. This returned I<without> a leading decimal point, but
may have leading or trailing zeroes.

=item * A{1,}

The millisecond of the day, based on the current time. In other words,
if it is 12:00:00.00, this returns 43200000.

=item * z{1,3}

The time zone short name.

=item * zzzz

The time zone long name.

=item * Z{1,3}

The time zone short name and the offset as one string, so something
like "CDT-0500".

=item * ZZZZ

The time zone long name.

=item * v{1,3}

The time zone short name.

=item * vvvv

The time zone long name.

=item * V{1,3}

The time zone short name.

=item * VVVV

The time zone long name.

=back

=head1 DateTime.pm and Storable

DateTime implements Storable hooks in order to reduce the size of a
serialized DateTime object.

=head1 KNOWN BUGS

The tests in F<20infinite.t> seem to fail on some machines,
particularly on Win32.  This appears to be related to Perl's internal
handling of IEEE infinity and NaN, and seems to be highly
platform/compiler/phase of moon dependent.

If you don't plan to use infinite datetimes you can probably ignore
this.  This will be fixed (somehow) in future versions.

=head1 SUPPORT

Support for this module is provided via the datetime@perl.org email
list. See http://datetime.perl.org/?MailingList for details.

Please submit bugs to the CPAN RT system at
http://rt.cpan.org/NoAuth/ReportBug.html?Queue=datetime or via email
at bug-datetime@rt.cpan.org.

=head1 DONATIONS

If you'd like to thank me for the work I've done on this module,
please consider making a "donation" to me via PayPal. I spend a lot of
free time creating free software, and would appreciate any support
you'd care to offer.

Please note that B<I am not suggesting that you must do this> in order
for me to continue working on this particular software. I will
continue to do so, inasmuch as I have in the past, for as long as it
interests me.

Similarly, a donation made in this way will probably not make me work
on this software much more, unless I get so many donations that I can
consider working on free software full time, which seems unlikely at
best.

To donate, log into PayPal and send money to autarch@urth.org or use
the button on this page:
L<http://www.urth.org/~autarch/fs-donation.html>

=head1 AUTHOR

Dave Rolsky <autarch@urth.org>

However, please see the CREDITS file for more details on who I really
stole all the code from.

=head1 COPYRIGHT

Copyright (c) 2003-2009 David Rolsky.  All rights reserved.  This
program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.

Portions of the code in this distribution are derived from other
works.  Please see the CREDITS file for more details.

The full text of the license can be found in the LICENSE file included
with this module.

=head1 SEE ALSO

datetime@perl.org mailing list

http://datetime.perl.org/

=cut