libc/time/tz-link.htm

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<title>Sources for Time Zone and Daylight Saving Time Data</title>
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 content="Sources of information about time zones and daylight saving time">
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<h1>Sources for Time Zone and Daylight Saving Time Data</h1>
<address>
@(#)tz-link.htm	8.32
</address>
<p>
This file is in the public domain, so clarified as of
2009-05-17 by Arthur David Olson.
</p>
<p>
Please send corrections to this web page to the
<a href="mailto:tz@elsie.nci.nih.gov">time zone mailing list</a>.</p>
<h2>The <code>tz</code> database</h2>
<p>
The <a href="http://en.wikipedia.org/wiki/Public_domain">public-domain</a>
time zone database contains code and data
that represent the history of local time
for many representative locations around the globe.
It is updated periodically to reflect changes made by political bodies
to <a href="http://en.wikipedia.org/wiki/Time_zone">time zone</a>
boundaries, <a
href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
title="Coordinated Universal Time">UTC</abbr></a> offsets, and
<a href="http://en.wikipedia.org/wiki/Daylight_saving">daylight-saving</a>
rules.
This database (often called <code>zoneinfo</code> or <a
href="http://en.wikipedia.org/wiki/TZ_database"><code>tz</code></a>)
is used by several implementations,
including
<a href="http://www.gnu.org/software/libc/">the
<abbr title="GNU's Not Unix">GNU</abbr>
C Library</a> used in
<a href="http://www.linux.org/"><abbr>GNU</abbr>/Linux</a>,
<a href="http://www.freebsd.org/">FreeBSD</a>,
<a href="http://netbsd.org/">NetBSD</a>,
<a href="http://openbsd.org/">OpenBSD</a>,
<a href="http://cygwin.com/">Cygwin</a>,
<a href="http://www.delorie.com/djgpp/"><abbr
title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
<a href="http://ibm.com/aix">AIX</a>,
<a href="http://www.apple.com/macosx/">Mac OS X</a>,
<a href="http://h71000.www7.hp.com/">OpenVMS</a>,
<a href="http://oracle.com/database">Oracle Database</a>,
<a href="http://sun.com/software/solaris">Solaris</a>,
<a href="http://h30097.www3.hp.com/">Tru64</a>, and
<a href="http://sco.com/products/unixware">UnixWare</a>.</p>
<p>
Each location in the database represents a national region where all
clocks keeping local time have agreed since 1970.
Locations are identified by continent or ocean and then by the name of
the location, which is typically the largest city within the region.
For example, <code>America/New_York</code>
represents most of the <abbr title="United States">US</abbr> eastern time zone;
<code>America/Phoenix</code> represents most of Arizona, which
uses mountain time without daylight saving time (<abbr
title="daylight saving time">DST</abbr>);
<code>America/Detroit</code> represents most of Michigan, which uses
eastern time but with different <abbr>DST</abbr> rules in 1975;
and other entries represent smaller regions like Starke County,
Indiana, which switched from central to eastern time in 1991
and switched back in 2006.
To use the database on an extended <a
href="http://en.wikipedia.org/wiki/POSIX"><abbr
title="Portable Operating System Interface">POSIX</abbr></a>
implementation set the <code>TZ</code> environment variable to
the location's full name, e.g., <code>TZ="America/New_York"</code>.</p>
<p>
In the <code>tz</code> database's
<a href="ftp://elsie.nci.nih.gov/pub"><abbr
title="File Transfer Protocol">FTP</abbr> distribution</a>
the code is in the file <code>tzcode<var>C</var>.tar.gz</code>,
where <code><var>C</var></code> is the code's version;
similarly, the data are in <code>tzdata<var>D</var>.tar.gz</code>,
where <code><var>D</var></code> is the data's version.
Each version is a four-digit year followed by lower-case letters
(a through z, then za through zz, then zza through zzz, and so on).
The following <a
href="http://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
these files to a <abbr>GNU</abbr>/Linux or similar host;
see the downloaded
<code>README</code> file for what to do next.</p>
<pre style="margin-left: 2em"><code>mkdir tz
cd tz
<a href="http://www.gnu.org/software/wget/">wget</a> 'ftp://elsie.nci.nih.gov/pub/tz*.tar.gz'
<a href="http://www.gnu.org/software/gzip/">gzip</a> -dc tzcode*.tar.gz | <a href="http://www.gnu.org/software/tar/">tar</a> -xf -
gzip -dc tzdata*.tar.gz | tar -xf -
</code></pre>
<p>
The code lets you compile the <code>tz</code> source files into
machine-readable binary files, one for each location. It also lets
you read a <code>tz</code> binary file and interpret time stamps for that
location.</p>
<p>
The data are by no means authoritative. If you find errors, please
send changes to the <a href="mailto:tz@elsie.nci.nih.gov">time zone
mailing list</a>. You can also <a
href="http://news.gmane.org/gmane.comp.time.tz">browse recent
messages</a> sent to the mailing list, <a
href="mailto:tz-request@elsie.nci.nih.gov">subscribe</a> to it,
retrieve the <a
href="ftp://elsie.nci.nih.gov/pub/tzarchive.gz">full archive of old
messages</a> (in gzip compressed format), or retrieve <a
href="ftp://munnari.oz.au/pub/oldtz">archived older versions of code
and data</a>.</p>
<p>
The Web has several other sources for time zone and daylight saving time data.
Here are some recent links that may be of interest.
</p>
<h2>Web pages using recent versions of the <code>tz</code> database</h2>
<p>
These are listed roughly in ascending order of complexity and fanciness.
</p>
<ul>
<li>
<a href="http://permatime.com">
Permatime</a>
is a service for generating and viewing links that refer to a
particular point in time and can be displayed in multiple timezones.
It uses the ruby tzinfo gem.
(From Tim Diggins, 2009-11-03.)
</li>
<li><a href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
lets you see the <code>TZ</code> values directly.</li>
<li><a
href="http://convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
Time in 1000 Places</a> uses descriptions of the values.</li>
<li><a href="http://www.timezoneconverter.com/">Time Zone Converter</a>
uses a pulldown menu.</li>
<li><a href="http://home.tiscali.nl/~t876506/TZworld.html">Complete
timezone information for all countries</a> displays tables of DST rules.
<li><a href="http://timeanddate.com/worldclock/">The World Clock -
Time Zones</a> lets you sort zone names and convert times.</li>
<li><a href="http://daylight-savings-time.info/">Graphical Display of
Time Zones and Daylight Saving Times</a> shows a graph of time
difference versus time for any pair of locations.</li>
<li>The <a href="http://worldtimeengine.com/">World Time Engine</a>
also contains data about time zone boundaries; it supports queries via place
names and shows location maps.</li>
<li><a href="http://simpletimerclocks.mozdev.org/">Simple Timer + Clocks</a>
is a Firefox add-on which uses a timezone data file generated from the
tz data files.</li>
</ul>
<h2>Other time zone database formats</h2>
<ul>
<li>The <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2445.txt">
Internet Calendaring and Scheduling Core Object Specification
(iCalendar)</a>, Internet <abbr title="Request For
Comments">RFC</abbr> 2445, published by the (now-concluded) <a
href="http://ietf.org/html.charters/OLD/calsch-charter.html"><abbr
title="Internet Engineering Task Force">IETF</abbr>
Calendaring and Scheduling Working Group (<abbr
title="Calendaring and Scheduling Working Group">calsch</abbr>)</a>
covers time zone
data; see its VTIMEZONE calendar component.
The <a href="http://calconnect.org/">Calendaring and Scheduling
Consortium</a> is promoting further work in this area. <a
href="http://calconnect.org/publications/icalendartimezoneproblemsandrecommendationsv1.0.pdf">iCalendar
TIMEZONE Problems and Recommendations</a> offers guidelines and
recommendations for the use of VTIMEZONE and <code>tz</code>.</li>
<li><a href="http://calconnect.org/dstlinks.shtml">Extended Daylight
Saving Time Links, Advisories and Changes</a> lists vendor material on recent
daylight saving time changes.</li>
<li><a
href="http://calconnect.org/publications/timezoneregistryandservicerecommendationsv1.0.pdf">Timezone
Registry and Service Recommendations</a> discusses a
strategy for defining and deploying a time zone
registration process that would establish unique names for each
version of each <code>tz</code> zone, along with a polygonal
representation of the geographical area corresponding to the
zone.</li>
<li>The <a
href="http://lists.w3.org/Archives/Public/www-rdf-calendar/">www-rdf-calendar</a>
list discusses <a
href="http://www.w3.org/RDF/"><abbr
title="Resource Description Framework">RDF</abbr></a>-based calendar
and group scheduling systems, and has a <a
href="http://www.w3.org/2002/12/cal/#tzd">workspace on time zone
data</a> converted from <code>tz</code>. An earlier <a
href="http://www.w3.org/2000/01/foo">schema</a> was sketched out.</li>
</ul>
<h2>Other <code>tz</code> compilers</h2>
<ul>
<li><a href="http://www.dachaplin.dsl.pipex.com/vzic/">Vzic iCalendar
Timezone Converter</a> describes a <a
href="http://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
program that compiles
<code>tz</code> source into iCalendar-compatible VTIMEZONE files.
Vzic is freely
available under the <a
href="http://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
General Public License (<abbr
title="General Public License">GPL</abbr>)</a>.</li>
<li><a href="http://sourceforge.net/projects/tzical">tziCal - tz
database conversion utility</a> is like Vzic, except for the <a
href="http://msdn.microsoft.com/netframework">.NET framework</a>.</li>
<li><a
href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
contains a script <code>parse_olson</code> that compiles
<code>tz</code> source into <a href="http://www.perl.org/">Perl</a>
modules. It is part of the Perl <a
href="http://datetime.perl.org/">DateTime Project</a>, which is freely
available under both the <abbr>GPL</abbr> and the Perl Artistic
License. DateTime::TimeZone also contains a script
<code>tests_from_zdump</code> that generates test cases for each clock
transition in the <code>tz</code> database.</li>
<li><a href="http://icu-project.org/">International Components for
Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
href="http://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
libraries for internationalization that
has a compiler from <code>tz</code> source
into an <abbr>ICU</abbr>-specific format.
<abbr>ICU</abbr> is freely available under a
<abbr title="Berkeley Software Distribution">BSD</abbr>-style license.</li>
<li><a href="http://joda-time.sourceforge.net/">Joda Time - Java date
and time <abbr title="Application Program Interface">API</abbr></a>
contains a class
<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
<code>tz</code> source into a Joda-specific binary format. Joda Time
is freely available under a <abbr>BSD</abbr>-style license.</li>
<li><a href="http://pytz.sourceforge.net">PyTZ - Python Time
Zone Library</a> compiles <code>tz</code> source into
<a href="http://python.org/">Python</a>.
It is freely available under a <abbr>BSD</abbr>-style license.</li>
<li><a href="http://tzinfo.rubyforge.org/">TZInfo - Ruby Timezone Library</a>
compiles <code>tz</code> source into
<a href="http://ruby-lang.org">Ruby</a>.
It is freely available under the <abbr
title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
<li>The <a href="http://chronos-st.org/">Chronos Date/Time
Library</a> is a <a href="http://smalltalk.org">Smalltalk</a> class
library that compiles <code>tz</code> source into a <a
href="http://date-time-zone.com/">time zone repository</a> whose format
is either proprietary or an <a href="http://www.w3.org/XML/"><abbr
title="Extensible Markup Language">XML</abbr></a>-encoded
representation.</li>
<li>Starting with version 8.5, <a href="http://tcl.tk/">Tcl</a>
contains a developer-oriented parser that compiles <samp>tz</samp>
source into text files, along with a runtime that can read those
files. Tcl is freely available under a <abbr>BSD</abbr>-style
license.</li>
</ul>
<h2>Other <code>tz</code> binary file readers</h2>
<ul>
<li>The <a
href="http://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
Library</a>
has an independent, thread-safe implementation of
a <code>tz</code> binary file reader.
This library is freely available under the
<a href="http://www.gnu.org/copyleft/lesser.html">
<abbr>GNU</abbr> Lesser General Public License
(<abbr title="Lesser General Public License">LGPL</abbr>)</a>,
and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
<li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
is a <code>tz</code> binary file reader written in Java.
It is freely available under the <abbr>LGPL</abbr>.</li>
<li>Tcl, mentioned above, also contains a
<code>tz</code> binary file reader.</li>
</ul>
<h2>Other <code>tz</code>-based time zone software</h2>
<ul>
<li><a href="http://stemhaus.com/firefox/foxclocks/">FoxClocks</a>
is an extension for <a
href="http://developer.mozilla.org/en/docs/Toolkit_API">Mozilla
Toolkit</a> applications like <a
href="http://mozilla.com/firefox">Firefox</a>, <a
href="http://mozilla.com/thunderbird">Thunderbird</a>, and
<a
href="http://www.mozilla.org/projects/calendar/sunbird/">Sunbird</a>.
It displays multiple clocks in the application window, and has a mapping
interface to <a href="http://earth.google.com/">Google Earth</a>.
It is freely available under the <abbr>GPL</abbr>.</li>
<li><a
href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
clock (intclock)</a> is a multi-timezone clock for
<abbr>GNU</abbr>/Linux and similar systems. It is freely available
under the <abbr>GPL</abbr>.</li>
<li><a href="http://codeplex.com/publicdomain">PublicDomain</a>
has a copy of a recent <code>tz</code> database, accessed via a <a
href="http://en.wikipedia.org/wiki/C_Sharp">C#</a> library. As its
name suggests, it is in the public domain. Only current time stamps
are well supported; historical data are compiled into the runtime but
are not easily accessible.</li>
<li><a href="http://java.sun.com/">Sun Java</a> releases since 1.4
contain a copy of a subset of a recent <code>tz</code> database in a
Java-specific format.</li>
<li><a href="http://kimmo.suominen.com/sw/timezone/">Time Zone</a> is
a <a href="http://wordpress.org/">WordPress</a> plugin. It is freely
available under a <abbr>BSD</abbr>-style license.</li>
<li><a
href="http://veladg.com/velaterra.html">VelaTerra</a> is
a Mac OS X program. Its developers
<a href="http://veladg.com/tzoffer.html">offer free
licenses</a> to <code>tz</code> contributors.</li>
<li><a
href="http://worldtimeexplorer.com/">World Time Explorer</a> is a
Microsoft Windows program.</li>
<li>
<a href="http://www.toriasoft.com">
WorldClock for Windows and Windows Mobile</a>
lets users "see the time in up to 25 locations in the world at once."
(From Hans Nieuwenhuis, 2009-11-02.)
</li>
<li>
<a href="http://www.relativedata.com/time-zone-master">
Time Zone Master Basic
</a> "allows people to display multiple desktop clocks, and to
research current and historical time information, as well as times of
astronomical events (sunrise/transit/set, moonrise/transit/set, phases,
season starts) for user-selected dates in the past and future. It can
automatically download, compile and use the tzdata**.gz database files
as they are released to keep the data up to date. The software is
free." (Davie Patte)
</li>
</ul>
<h2>Other time zone databases</h2>
<ul>
<li><a href="http://www.astro.com/cgi/aq.cgi">Atlas Query</a>
is Astrodienst's Web version of Shanks's
excellent time zone history atlases published in both <a
href="http://astrocom.com/products/software.php?software_id=ibmwboth">computer</a>
and book form (<a
href="http://astrocom.com/products/book.php?book_id=b110x">one volume
for the USA</a>, and <a
href="http://astrocom.com/products/book.php?book_id=b112x">one for
other locations</a>) by <a
href="http://astrocom.com/">Astro Communications Services</a>.</li>
<li><a href="http://worldtime.com/">WORLDTIME: interactive atlas,
time info, public holidays</a>
contains information on local time, sunrise and sunset,
and public holidays in several hundred cities around the world.</li>
<li><a href="http://worldtimeserver.com/">World Time Server</a>
is another time zone database.</li>
<li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
contains data from the Time Service Department of the
<abbr>US</abbr> Naval Observatory, used as the source
for the <code>usno*</code> files in the <code>tz</code> distribution.</li>
<li>The <a href="http://iata.org/ps/publications/SSIM.htm">Standard
Schedules Information Manual</a> of the
<a href="http://iata.org/index.htm">International Air Transport
Association</a>
gives current time zone rules for airports served by commercial aviation.</li>
<li>Some Microsoft Windows versions contain time zone information in
an undocumented format, with IDs that can be mapped to <code>TZ</code>
values using the <a
href="http://unicode.org/cldr/data/diff/supplemental/windows_tzid.html">Windows
&rarr; Tzid table</a> maintained by the <abbr
title="Common Locale Data Repository">CLDR</abbr> data mentioned
below.</li>
<li>
<a href="http://code.google.com/p/tzdata/">
http://code.google.com/p/tzdata/
</a>
provides programming-language-specific representations of timezone
data. Currently this includes XML, PHP, Ruby, Javascript, JSON and CSV
formatted data. The repository is updated as soon as the FTP
distribution is updated. All data can be downloaded as a zip and/or it
can be obtained/synced via anonymous SVN. Data is made available under
the MIT license. (From Rich Tibbett.)
</li>
</ul>
<h2>Maps</h2>
<ul>
<li>The <a href="https://www.cia.gov/">United States Central
Intelligence Agency (<abbr
title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
href="https://www.cia.gov/library/publications/the-world-factbook/reference_maps/pdf/time_zones.pdf">time
zone map</a>; the
<a
href="http://www.lib.utexas.edu/maps/world.html">Perry-Casta&ntilde;eda
Library Map Collection</a>
of the University of Texas at Austin has copies of
recent editions.
The pictorial quality is good,
but the maps do not indicate summer time,
and parts of the data are a few years out of date.</li>
<li><a href="http://worldtimezone.com/">Current time around the world
and standard time zones map of the world</a>
has several fancy time zone maps; it covers Russia particularly well.
The maps' pictorial quality is not quite as good as the
<abbr>CIA</abbr>'s
but the maps are more up to date.</li>
</ul>
<h2>Time zone boundaries</h2>
<ul>
<li><a href="http://efele.net/maps/tz/">TZ timezone maps</a> contains a <a
href="http://en.wikipedia.org/wiki/Shapefile">shapefile</a> of the
<code>tz</code> regions in the world.</li>
<li><a href="http://statoids.com/statoids.html">Administrative Divisions
of Countries ("Statoids")</a> contains detailed lists of
<code>tz</code>-related zone subdivision data.</li>
<li><a href="http://home.tiscali.nl/~t876506/Multizones.html">Time
zone boundaries for multizone countries</a> summarizes legal
boundaries between time zones within countries.</li>
<li>Manifold.net's <a
href="http://www.manifold.net/download/freemaps.html">Free Maps and
<abbr title="Geographic Information Systems">GIS</abbr>
Data</a> includes a Manifold-format map of
world time zone boundaries distributed under the
<abbr>GPL</abbr>.</li>
<li>The <abbr>US</abbr> Geological Survey's National Atlas of
the United States
publishes the <a href="http://nationalatlas.gov/mld/timeznp.html">Time
Zones of the United States</a> in the public domain.</li>
<li>The GeoCommunity lists several commercial sources for <a
href="http://spatialnews.geocomm.com/features/timezones/">International
Time Zones and Time Zone Data</a>.</li>
<li>A ship within the <a
href="http://en.wikipedia.org/wiki/Territorial_waters">territorial
waters</a> of any nation uses that nation's time. In international
waters, time zone boundaries are meridians 15&deg; apart, except that
UTC&minus;12 and UTC+12 are each 7.5&deg; wide and are separated by
the 180&deg; meridian (not by the International Date Line, which is
for land and territorial waters only). A captain can change ship's
clocks any time after entering a new time zone; midnight changes are
common.</li>
</ul>
<h2>Civil time concepts and history</h2>
<ul>
<li><a href="http://physics.nist.gov/GenInt/Time/time.html">A
Walk through Time</a>
surveys the evolution of timekeeping.</li>
<li><a href="http://webexhibits.org/daylightsaving/">About Daylight
Saving Time - History, rationale, laws &amp; dates</a>
is an overall history of <abbr>DST</abbr>.</li>
<li><a href="http://energy.ca.gov/daylightsaving.html">Saving Time,
Saving Energy</a> discusses a primary justification for <abbr>DST</abbr>.</li>
<li><a href="http://seizethedaylight.com/dst/">Who Knew? A Brief
History of Daylight Saving Time</a> summarizes some of the contentious
history of <abbr>DST</abbr>.</li>
<li><a href="http://toi.iriti.cnr.it/">The
Time of Internet</a>
describes time zones and daylight saving time,
with diagrams.
The time zone map is out of date, however.</li>
<li><a href="http://www.phys.uu.nl/~vgent/idl/idl.htm">A History of
the International Date Line</a> tells the story of the most important
time zone boundary.</li>
<li><a href="http://statoids.com/tconcept.html">Basic Time
Zone Concepts</a> discusses terminological issues behind time zones.</li>
</ul>
<h2>National histories of legal time</h2>
<dl>
<dt>Australia</dt>
<dd>The Parliamentary Library has commissioned <a
href="http://www.aph.gov.au/LIBRARY/Pubs/rn/2006-07/07rn13.pdf">research
note on daylight saving time in Australia</a>.
The Bureau of Meteorology publishes a list of
<a href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation Dates of Daylight Savings Time within Australia</a>.</dd>
<dt>Belgium</dt>
<dd>The Royal Observatory of Belgium maintains a table of <a
href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
<dt>Brazil</dt>
<dd>The Time Service Department of the National Observatory
records <a href="http://pcdsh01.on.br/DecHV.html"
hreflang="pt-BR">Brazil's daylight saving time decrees (in
Portuguese)</a>.</dd>
<dt>Canada</dt>
<dd>The Institute for National Measurement Standards publishes current
and some older information about <a
href="http://inms-ienm.nrc-cnrc.gc.ca/time_services/daylight_saving_e.html">Time
Zones &amp; Daylight Saving Time</a>.</dd>
<dt>Chile</dt>
<dd>The Chilean Hydrographic and Oceanographic Service publishes a <a
href="http://www.horaoficial.cl/horaof.htm" hreflang="es"> history of
official time (in Spanish)</a>.</dd>
<dt>Germany</dt>
<dd>The National Institute for Science and Technology maintains the <a
href="http://www.ptb.de/en/org/4/44/441/dars_e.htm">Realisation of
Legal Time in Germany</a>.</dd>
<dt>Israel</dt>
<dd>The Interior Ministry periodically issues <a
href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
hreflang="he">announcements (in Hebrew)</a>.</dd>
<dt>Mexico</dt>
<dd>The Investigation and Analysis Service of the Mexican Library of
Congress has published a <a
href="http://www.cddhcu.gob.mx/bibliot/publica/inveyana/polisoc/horver/"
hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
<dt>Malaysia</dt>
<dd>See Singapore below.</dd>
<dt>Netherlands</dt>
<dd><a href="http://www.phys.uu.nl/~vgent/wettijd/wettijd.htm"
hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
covers the history of local time in the Netherlands from ancient times.</dd>
<dt>New Zealand</dt>
<dd>The Department of Internal Affairs maintains a brief history <a
href="http://dia.govt.nz/diawebsite.nsf/wpg_URL/Resource-material-Information-We-Provide-About-Daylight-Saving">About
Daylight Saving</a>. The privately-maintained <a
href="http://astrologyschool.com/nztime.html">History of New Zealand
time</a> has more details.</dd>
<dt>Norway</dt>
<dd>The Norwegian Meteorological Institute lists
<a href="http://met.no/met/met_lex/q_u/sommertid.html" hreflang="no">Summer
time in Norway (in Norwegian)</a>, citing the
Institute of Theoretical Astrophysics, Oslo.</dd>
<dt>Singapore</dt>
<dd><a
href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
is Singapore in the "Wrong" Time Zone?</a> details the
history of legal time in Singapore and Malaysia.</dd>
<dt>United Kingdom</dt>
<dd><a
href="http://www.srcf.ucam.org/~jsm28/british-time/">History of
legal time in Britain</a> discusses in detail the country
with perhaps the best-documented history of clock adjustments.
The National Physical Laboratory also maintains an <a
href="http://www.npl.co.uk/server.php?show=ConWebDoc.2714">Archive
of Summer time dates</a>.</dd>
</dl>
<h2>Precision timekeeping</h2>
<ul>
<li><a
href="http://literature.agilent.com/litweb/pdf/5965-7984E.pdf">The
Science of Timekeeping</a> is a thorough introduction
to the theory and practice of precision timekeeping.</li>
<li><a href="http://www.ntp.org/"><abbr
title="Network Time Protocol">NTP</abbr>: The Network
Time Protocol</a>
discusses how to synchronize clocks of
Internet hosts.</li>
<li><a
href="ftp://ftp.rfc-editor.org/in-notes/rfc4833.txt">Timezone
Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
(Internet <abbr>RFC</abbr> 4833)
specifies a <a
href="http://www.dhcp.org/">DHCP</a> option for a server to configure
a client's time zone and daylight saving settings automatically.</li>
<li><a href="http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.html">A Few
Facts Concerning <abbr title="Greenwich Mean Time">GMT</abbr>, <abbr
title="Universal Time">UT</abbr>, and
the <abbr title="Royal Greenwich Observatory">RGO</abbr></a>
answers questions like "What is the
difference between <abbr>GMT</abbr> and <abbr>UTC</abbr>?"</li>
<li><a
href="http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
Times</a> explains more abstruse astronomical time scales like
<abbr title="Terrestrial Dynamic Time">TDT</abbr>,
<abbr title="Geocentric Coordinate Time">TCG</abbr>, and
<abbr title="Barycentric Dynamic Time">TDB</abbr>.
<a href="http://www.ucolick.org/~sla/leapsecs/timescales.html">Time
Scales</a> goes into more detail, particularly for historical variants.</li>
<li>The <a href="http://iau.org/"><abbr
title="International Astronomical Union">IAU</abbr></a>'s <a
href="http://www.iau-sofa.rl.ac.uk/"><abbr
title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
initiative publishes Fortran
code for converting among time scales like
<abbr title="International Atomic Time">TAI</abbr>,
<abbr>TDB</abbr>, <abbr>TDT</abbr> and
<abbr>UTC</abbr>.</li>
<li><a href="http://jpl.nasa.gov/basics/bsf2-3.htm">Basics of
Space Flight - Reference Systems - Time Conventions</a>
briefly explains interplanetary space flight timekeeping.</li>
<li><a
href="http://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly
describes Mars Coordinated Time (<abbr
title="Mars Coordinated Time">MTC</abbr>) and the
diverse local time
scales used by each landed mission on Mars.</li>
<li><a href="http://leapsecond.com/">LeapSecond.com</a> is
dedicated not only to leap seconds but to precise time and frequency
in general. It covers the state of the art in amateur timekeeping, and
how the art has progressed over the past few decades.</li>
<li><a
href="http://hpiers.obspm.fr/eop-pc/products/bulletins/bulletins.html">Bulletins
maintained by the
<abbr title="International Earth Rotation Service">IERS</abbr>
<abbr title="Earth Orientation Parameters">EOP</abbr>
(<abbr title="Product Center">PC</abbr>)</a> contains official publications of
the Earth Orientation Parameters Product Center of the
International Earth Rotation Service, the committee that decides
when leap seconds occur.</li>
<li>The <a
href="http://six.pairlist.net/mailman/listinfo/leapsecs">Leap
Second Discussion List</a> covers <a
href="http://gauss.gge.unb.ca/papers.pdf/gpsworld.november99.pdf">McCarthy
and Klepczynski's proposal to discontinue leap seconds</a>,
discussed further in
<a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
leap second: its history and possible future</a>.
The (now disbanded) <a href="http://members.aas.org/comms/leap.cfm"><abbr
title="American Astronomical Society">AAS</abbr> Leap Second
Committee</a> has solicited input on this proposal.
<a href="http://www.ucolick.org/~sla/leapsecs/">The
Future of Leap Seconds</a> covers this
contentious issue.</li>
</ul>
<h2>Time notation</h2>
<ul>
<li>
<a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A Summary of
the International Standard Date and Time Notation</a> is a good
summary of
<a
href="http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=40874"><abbr
title="International Organization for Standardization">ISO</abbr>
8601:2004 -- Data elements and interchange formats -- Information
interchange -- Representation of dates and times</a>.</li>
<li>
<a href="http://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
Schema: Datatypes - dateTime</a> specifies a format inspired by
<abbr>ISO</abbr> 8601 that is in common use in XML data.</li>
<li>
<a href="ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt">Internet
Message Format</a> (Internet <abbr>RFC</abbr> 2822) &sect;3.3
specifies the time notation used in email and <a
href="ftp://ftp.rfc-editor.org/in-notes/rfc2616.txt"><abbr>HTTP</abbr></a>
headers.</li>
<li>
<a href="ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt">Date and Time
on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
specifies an <abbr>ISO</abbr> 8601
profile for use in new Internet
protocols.</li>
<li>
<a href="http://www.hackcraft.net/web/datetime/">Date &amp; Time
Formats on the Web</a> surveys web- and Internet-oriented date and time
formats.</li>
<li>
<a href="http://exit109.com/~ghealton/y2k/yrexamples.html">The
Best of Dates, the Worst of Dates</a> covers many problems encountered
by software developers when handling dates and time stamps.</li>
<li>The <a
href="http://unicode.org/cldr/">Unicode Common Locale Data Repository
(<abbr>CLDR</abbr>) Project</a> has localizations for time zone names,
abbreviations, identifiers, and formats. For example, it contains
French translations for "Eastern European Summer Time", "<abbr
title="Eastern European Summer Time">EEST</abbr>", and
"Bucharest". <a
href="http://unicode.org/cldr/data/charts/by_type/names.metazone.html">By-Type
Chart: names.metazone</a> shows these values for many locales.
<abbr>ICU</abbr> contains a mechanism for using this data.</li>
<li>Alphabetic time zone abbreviations should not be used as unique
identifiers for <abbr>UTC</abbr> offsets as they are ambiguous in
practice. For example, "<abbr>EST</abbr>" denotes 5 hours behind
<abbr>UTC</abbr> in English-speaking North America, but it denotes 10
or 11 hours ahead of <abbr>UTC</abbr> in Australia; and
French-speaking North Americans prefer
"<abbr title="Heure Normale de l'Est">HNE</abbr>" to
"<abbr>EST</abbr>". For <abbr>POSIX</abbr> the <code>tz</code>
database contains English abbreviations for all time stamps but in
many cases these are merely inventions of the database
maintainers.</li>
<li>Numeric time zone abbreviations typically count hours east of
<abbr>UTC</abbr>, e.g., <code>+09</code> for Japan and
<code>-10</code> for Hawaii. However, the <abbr>POSIX</abbr>
<code>TZ</code> environment variable uses the opposite convention. For
example, one might use <code>TZ="JST-9"</code> and
<code>TZ="HST10"</code> for Japan and Hawaii, respectively. If the
<code>tz</code> database is available, it is usually better to use
settings like <code>TZ="Asia/Tokyo"</code> and
<code>TZ="Pacific/Honolulu"</code> instead, as this should avoid
confusion, handle old time stamps better, and insulate you better from
any future changes to the rules. One should never set
<abbr>POSIX</abbr> <code>TZ</code> to a value like
<code>"GMT-9"</code>, though, since this would falsely claim that
local time is nine hours ahead of <abbr>UTC</abbr> and the time zone
is called "<abbr>GMT</abbr>".</li>
</ul>
<h2>Related indexes</h2>
<ul>
<li><a href="tz-art.htm">Time and the Arts</a></li>
<li><a href="http://www.dmoz.org/Reference/Time/">Open Directory -
Reference: Time</a></li>
<li><a href="http://directory.google.com/Top/Reference/Time/">Google Directory
- Reference &gt; Time</a></li>
<li><a href="http://dir.yahoo.com/Science/Measurements_and_Units/Time">Yahoo!
Directory &gt; Science &gt; Measurements and Units &gt; Time</a></li>
</ul>
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