1# Generate zic format 'leapseconds' from NIST/IERS format 'leap-seconds.list'.
2
3# This file is in the public domain.
4
5# This program uses awk arithmetic.  POSIX requires awk to support
6# exact integer arithmetic only through 10**10, which means for NTP
7# timestamps this program works only to the year 2216, which is the
8# year 1900 plus 10**10 seconds.  However, in practice
9# POSIX-conforming awk implementations invariably use IEEE-754 double
10# and so support exact integers through 2**53.  By the year 2216,
11# POSIX will almost surely require at least 2**53 for awk, so for NTP
12# timestamps this program should be good until the year 285,428,681
13# (the year 1900 plus 2**53 seconds).  By then leap seconds will be
14# long obsolete, as the Earth will likely slow down so much that
15# there will be more than 25 hours per day and so some other scheme
16# will be needed.
17
18BEGIN {
19  print "# Allowance for leap seconds added to each time zone file."
20  print ""
21  print "# This file is in the public domain."
22  print ""
23  print "# This file is generated automatically from the data in the public-domain"
24  print "# NIST/IERS format leap-seconds.list file, which can be copied from"
25  print "# <https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list>"
26  print "# or, in a variant with different comments, from"
27  print "# <ftp://ftp.boulder.nist.gov/pub/time/leap-seconds.list>."
28  print "# For more about leap-seconds.list, please see"
29  print "# The NTP Timescale and Leap Seconds"
30  print "# <https://www.eecis.udel.edu/~mills/leap.html>."
31  print ""
32  print "# The rules for leap seconds are specified in Annex 1 (Time scales) of:"
33  print "# Standard-frequency and time-signal emissions."
34  print "# International Telecommunication Union - Radiocommunication Sector"
35  print "# (ITU-R) Recommendation TF.460-6 (02/2002)"
36  print "# <https://www.itu.int/rec/R-REC-TF.460-6-200202-I/>."
37  print "# The International Earth Rotation and Reference Systems Service (IERS)"
38  print "# periodically uses leap seconds to keep UTC to within 0.9 s of UT1"
39  print "# (a proxy for Earth's angle in space as measured by astronomers)"
40  print "# and publishes leap second data in a copyrighted file"
41  print "# <https://hpiers.obspm.fr/iers/bul/bulc/Leap_Second.dat>."
42  print "# See: Levine J. Coordinated Universal Time and the leap second."
43  print "# URSI Radio Sci Bull. 2016;89(4):30-6. doi:10.23919/URSIRSB.2016.7909995"
44  print "# <https://ieeexplore.ieee.org/document/7909995>."
45  print ""
46  print "# There were no leap seconds before 1972, as no official mechanism"
47  print "# accounted for the discrepancy between atomic time (TAI) and the earth's"
48  print "# rotation.  The first (\"1 Jan 1972\") data line in leap-seconds.list"
49  print "# does not denote a leap second; it denotes the start of the current definition"
50  print "# of UTC."
51  print ""
52  print "# All leap-seconds are Stationary (S) at the given UTC time."
53  print "# The correction (+ or -) is made at the given time, so in the unlikely"
54  print "# event of a negative leap second, a line would look like this:"
55  print "# Leap	YEAR	MON	DAY	23:59:59	-	S"
56  print "# Typical lines look like this:"
57  print "# Leap	YEAR	MON	DAY	23:59:60	+	S"
58
59  monthabbr[ 1] = "Jan"
60  monthabbr[ 2] = "Feb"
61  monthabbr[ 3] = "Mar"
62  monthabbr[ 4] = "Apr"
63  monthabbr[ 5] = "May"
64  monthabbr[ 6] = "Jun"
65  monthabbr[ 7] = "Jul"
66  monthabbr[ 8] = "Aug"
67  monthabbr[ 9] = "Sep"
68  monthabbr[10] = "Oct"
69  monthabbr[11] = "Nov"
70  monthabbr[12] = "Dec"
71
72  sstamp_init()
73}
74
75# In case the input has CRLF form a la NIST.
76{ sub(/\r$/, "") }
77
78/^#[ \t]*[Uu]pdated through/ || /^#[ \t]*[Ff]ile expires on/ {
79    last_lines = last_lines $0 "\n"
80}
81
82/^#[$][ \t]/ { updated = $2 }
83/^#[@][ \t]/ { expires = $2 }
84
85/^[ \t]*#/ { next }
86
87{
88    NTP_timestamp = $1
89    TAI_minus_UTC = $2
90    if (old_TAI_minus_UTC) {
91	if (old_TAI_minus_UTC < TAI_minus_UTC) {
92	    sign = "23:59:60\t+"
93	} else {
94	    sign = "23:59:59\t-"
95	}
96	sstamp_to_ymdhMs(NTP_timestamp - 1, ss_NTP)
97	printf "Leap\t%d\t%s\t%d\t%s\tS\n", \
98	  ss_year, monthabbr[ss_month], ss_mday, sign
99    }
100    old_TAI_minus_UTC = TAI_minus_UTC
101}
102
103END {
104    print ""
105
106    if (expires) {
107      sstamp_to_ymdhMs(expires, ss_NTP)
108
109      print "# UTC timestamp when this leap second list expires."
110      print "# Any additional leap seconds will come after this."
111      if (! EXPIRES_LINE) {
112	print "# This Expires line is commented out for now,"
113	print "# so that pre-2020a zic implementations do not reject this file."
114      }
115      printf "%sExpires %.4d\t%s\t%.2d\t%.2d:%.2d:%.2d\n", \
116	EXPIRES_LINE ? "" : "#", \
117	ss_year, monthabbr[ss_month], ss_mday, ss_hour, ss_min, ss_sec
118    } else {
119      print "# (No Expires line, since the expires time is unknown.)"
120    }
121
122    # The difference between the NTP and POSIX epochs is 70 years
123    # (including 17 leap days), each 24 hours of 60 minutes of 60
124    # seconds each.
125    epoch_minus_NTP = ((1970 - 1900) * 365 + 17) * 24 * 60 * 60
126
127    print ""
128    print "# POSIX timestamps for the data in this file:"
129    if (updated) {
130      sstamp_to_ymdhMs(updated, ss_NTP)
131      printf "#updated %d (%.4d-%.2d-%.2d %.2d:%.2d:%.2d UTC)\n", \
132	updated - epoch_minus_NTP, \
133	ss_year, ss_month, ss_mday, ss_hour, ss_min, ss_sec
134    } else {
135      print "#(updated time unknown)"
136    }
137    if (expires) {
138      sstamp_to_ymdhMs(expires, ss_NTP)
139      printf "#expires %d (%.4d-%.2d-%.2d %.2d:%.2d:%.2d UTC)\n", \
140	expires - epoch_minus_NTP, \
141	ss_year, ss_month, ss_mday, ss_hour, ss_min, ss_sec
142    } else {
143      print "#(expires time unknown)"
144    }
145    printf "\n%s", last_lines
146}
147
148# sstamp_to_ymdhMs - convert seconds timestamp to date and time
149#
150# Call as:
151#
152#    sstamp_to_ymdhMs(sstamp, epoch_days)
153#
154# where:
155#
156#    sstamp - is the seconds timestamp.
157#    epoch_days - is the timestamp epoch in Gregorian days since 1600-03-01.
158#	ss_NTP is appropriate for an NTP sstamp.
159#
160# Both arguments should be nonnegative integers.
161# On return, the following variables are set based on sstamp:
162#
163#    ss_year	- Gregorian calendar year
164#    ss_month	- month of the year (1-January to 12-December)
165#    ss_mday	- day of the month (1-31)
166#    ss_hour	- hour (0-23)
167#    ss_min	- minute (0-59)
168#    ss_sec	- second (0-59)
169#    ss_wday	- day of week (0-Sunday to 6-Saturday)
170#
171# The function sstamp_init should be called prior to using sstamp_to_ymdhMs.
172
173function sstamp_init()
174{
175  # Days in month N, where March is month 0 and January month 10.
176  ss_mon_days[ 0] = 31
177  ss_mon_days[ 1] = 30
178  ss_mon_days[ 2] = 31
179  ss_mon_days[ 3] = 30
180  ss_mon_days[ 4] = 31
181  ss_mon_days[ 5] = 31
182  ss_mon_days[ 6] = 30
183  ss_mon_days[ 7] = 31
184  ss_mon_days[ 8] = 30
185  ss_mon_days[ 9] = 31
186  ss_mon_days[10] = 31
187
188  # Counts of days in a Gregorian year, quad-year, century, and quad-century.
189  ss_year_days = 365
190  ss_quadyear_days = ss_year_days * 4 + 1
191  ss_century_days = ss_quadyear_days * 25 - 1
192  ss_quadcentury_days = ss_century_days * 4 + 1
193
194  # Standard day epochs, suitable for epoch_days.
195  # ss_MJD = 94493
196  # ss_POSIX = 135080
197  ss_NTP = 109513
198}
199
200function sstamp_to_ymdhMs(sstamp, epoch_days, \
201			  quadcentury, century, quadyear, year, month, day)
202{
203  ss_hour = int(sstamp / 3600) % 24
204  ss_min = int(sstamp / 60) % 60
205  ss_sec = sstamp % 60
206
207  # Start with a count of days since 1600-03-01 Gregorian.
208  day = epoch_days + int(sstamp / (24 * 60 * 60))
209
210  # Compute a year-month-day date with days of the month numbered
211  # 0-30, months (March-February) numbered 0-11, and years that start
212  # start March 1 and end after the last day of February.  A quad-year
213  # starts on March 1 of a year evenly divisible by 4 and ends after
214  # the last day of February 4 years later.  A century starts on and
215  # ends before March 1 in years evenly divisible by 100.
216  # A quad-century starts on and ends before March 1 in years divisible
217  # by 400.  While the number of days in a quad-century is a constant,
218  # the number of days in each other time period can vary by 1.
219  # Any variation is in the last day of the time period (there might
220  # or might not be a February 29) where it is easy to deal with.
221
222  quadcentury = int(day / ss_quadcentury_days)
223  day -= quadcentury * ss_quadcentury_days
224  ss_wday = (day + 3) % 7
225  century = int(day / ss_century_days)
226  century -= century == 4
227  day -= century * ss_century_days
228  quadyear = int(day / ss_quadyear_days)
229  day -= quadyear * ss_quadyear_days
230  year = int(day / ss_year_days)
231  year -= year == 4
232  day -= year * ss_year_days
233  for (month = 0; month < 11; month++) {
234    if (day < ss_mon_days[month])
235      break
236    day -= ss_mon_days[month]
237  }
238
239  # Convert the date to a conventional day of month (1-31),
240  # month (1-12, January-December) and Gregorian year.
241  ss_mday = day + 1
242  if (month <= 9) {
243    ss_month = month + 3
244  } else {
245    ss_month = month - 9
246    year++
247  }
248  ss_year = 1600 + quadcentury * 400 + century * 100 + quadyear * 4 + year
249}
250