[tz-announce] 2021b release of tz code and data available

Paul Eggert eggert at cs.ucla.edu
Sat Sep 25 00:58:09 UTC 2021

The 2021b release of the tz code and data is available.

Partly because it's been so long since 2021a, this release has many 
changes, noted below. In one area, noted "Merge more location-based 
Zones" below, the mailing list has had significant trouble coming to a 
long-term consensus. Because we should publish something before the 
Samoa change takes effect, this release contains just one step towards 
making tzdb fairer and more equitable in future releases.

This release reflects the following changes, which were either 
circulated on the tz mailing list or are relatively minor technical or 
administrative changes:

     Jordan now starts DST on February's last Thursday.
     Samoa no longer observes DST.
     Merge more location-based Zones whose timestamps agree since 1970.
     Move some backward-compatibility links to 'backward'.
     Rename Pacific/Enderbury to Pacific/Kanton.
     Correct many pre-1993 transitions in Malawi, Portugal, etc.
     zic now creates each output file or link atomically.
     zic -L no longer omits the POSIX TZ string in its output.
     zic fixes for truncation and leap second table expiration.
     zic now follows POSIX for TZ strings using all-year DST.
     Fix some localtime crashes and bugs in obscure cases.
     zdump -v now outputs more-useful boundary cases.
     tzfile.5 better matches a draft successor to RFC 8536.
     A new file SECURITY.

   This release is prompted by recent announcements by Jordan and Samoa.
   It incorporates many other changes that had accumulated since 2021a.
   However, it omits most proposed changes that merged all Zones
   agreeing since 1970, as concerns were raised about doing too many of
   these changes at once.  It does keeps some of these changes in the
   interest of making tzdb more equitable one step at a time; see
   "Merge more location-based Zones" below.

   Changes to future timestamps

     Jordan now starts DST on February's last Thursday.
     (Thanks to Steffen Thorsen.)

     Samoa no longer observes DST.  (Thanks to Geoffrey D. Bennett.)

   Changes to zone name

     Rename Pacific/Enderbury to Pacific/Kanton.  When we added
     Enderbury in 1993, we did not know that it is uninhabited and that
     Kanton (population two dozen) is the only inhabited location in
     that timezone.  The old name is now a backward-compatility link.

   Changes to past timestamps

     Correct many pre-1993 transitions, fixing entries originally
     derived from Shanks, Whitman, and Mundell.  The fixes include:
       - Barbados: standard time was introduced in 1911, not 1932; and
	DST was observed in 1942-1944
       - Cook Islands: In 1899 they switched from east to west of GMT,
	celebrating Christmas for two days.  They (and Niue) switched
	to standard time in 1952, not 1901.
       - Guyana: corrected LMT for Georgetown; the introduction of
	standard time in 1911, not 1915; and corrections to 1975 and
	1992 transitions
       - Kanton: uninhabited before 1937-08-31
       - Niue: only observed -11:20 from 1952 through 1964, then went to
         -11 instead of -11:30
       - Portugal: DST was observed in 1950
       - Tonga: corrected LMT; the introduction of standard time in 1945,
         not 1901; and corrections to the transition from +12:20 to +13
         in 1961, not 1941
     Additional fixes to entries in the 'backzone' file include:
       - Enderbury: inhabited only 1860/1885 and 1938-03-06/1942-02-09
       - The Gambia: 1933 and 1942 transitions
       - Malawi: several 1911 through 1925 transitions
       - Sierra Leone: several 1913 through 1941 transitions, and DST
	was NOT observed in 1957 through 1962
     (Thanks to P Chan, Michael Deckers, Alexander Krivenyshev and
     Alois Treindl.)

     Merge more location-based Zones whose timestamps agree since 1970,
     as pre-1970 timestamps are out of scope.  This is part of a
     process that has been ongoing since 2013.  This does not affect
     post-1970 timestamps, and timezone historians who build with 'make
     PACKRATDATA=backzone' should see no changes to pre-1970 timestamps.
     When merging, keep the most-populous location's data, and move
     data for other locations to 'backzone' with a backward
     link in 'backward'.  For example, move America/Creston data to
     'backzone' with a link in 'backward' from America/Phoenix because
     the two timezones' timestamps agree since 1970; this change
     affects some pre-1968 timestamps in America/Creston because
     Creston and Phoenix disagreed before 1968.  The affected Zones
     are Africa/Accra, America/Atikokan, America/Blanc-Sablon,
     America/Creston, America/Curacao, America/Nassau,
     America/Port_of_Spain, Antarctica/DumontDUrville, and

   Changes to maintenance procedure

     The new file SECURITY covers how to report security-related bugs.

     Several backward-compatibility links have been moved to the
     'backward' file.  These links, which range from Africa/Addis_Ababa
     to Pacific/Saipan, are only for compatibility with now-obsolete
     guidelines suggesting an entry for every ISO 3166 code.
     The intercontinental convenience links Asia/Istanbul and
     Europe/Nicosia have also been moved to 'backward'.

   Changes to code

     zic now creates each output file or link atomically,
     possibly by creating a temporary file and then renaming it.
     This avoids races where a TZ setting would temporarily stop
     working while zic was installing a replacement file or link.

     zic -L no longer omits the POSIX TZ string in its output.
     Starting with 2020a, zic -L truncated its output according to the
     "Expires" directive or "#expires" comment in the leapseconds file.
     The resulting TZif files omitted daylight saving transitions after
     the leap second table expired, which led to far less-accurate
     predictions of times after the expiry.  Although future timestamps
     cannot be converted accurately in the presence of leap seconds, it
     is more accurate to convert near-future timestamps with a few
     seconds error than with an hour error, so zic -L no longer
     truncates output in this way.

     Instead, when zic -L is given the "Expires" directive, it now
     outputs the expiration by appending a no-change entry to the leap
     second table.  Although this should work well with most TZif
     readers, it does not conform to Internet RFC 8536 and some pickier
     clients (including tzdb 2017c through 2021a) reject it, so
     "Expires" directives are currently disabled by default.  To enable
     them, set the EXPIRES_LINE Makefile variable.  If a TZif file uses
     this new feature it is marked with a new TZif version number 4,
     a format intended to be documented in a successor to RFC 8536.

     zic -L LEAPFILE -r @LO no longer generates an invalid TZif file
     that omits leap second information for the range LO..B when LO
     falls between two leap seconds A and B.  Instead, it generates a
     TZif version 4 file that represents the previously-missing

     The TZif reader now allows the leap second table to begin with a
     correction other than -1 or +1, and to contain adjacent
     transitions with equal corrections.  This supports TZif version 4.

     The TZif reader now lets leap seconds occur less than 28 days
     apart.  This supports possible future TZif extensions.

     Fix bug that caused 'localtime' etc. to crash when TZ was
     set to a all-year DST string like "EST5EDT4,0/0,J365/25" that does
     not conform to POSIX but does conform to Internet RFC 8536.

     Fix another bug that caused 'localtime' etc. to crash when TZ was
     set to a POSIX-conforming but unusual TZ string like
     "EST5EDT4,0/0,J365/0", where almost all the year is DST.

     Fix yet another bug that caused 'localtime' etc. to mishandle slim
     TZif files containing leap seconds after the last explicit
     transition in the table, or when handling far-future timestamps
     in slim TZif files lacking leap seconds.

     Fix localtime misbehavior involving positive leap seconds.
     This change affects only behavior for "right" system time,
     which contains leap seconds, and only if the UT offset is
     not a multiple of 60 seconds when a positive leap second occurs.
     (No such timezone exists in tzdb, luckily.)  Without the fix,
     the timestamp was ambiguous during a positive leap second.
     With the fix, any seconds occurring after a positive leap second
     and within the same localtime minute are counted through 60, not
     through 59; their UT offset (tm_gmtoff) is the same as before.
     Here is how the fix affects timestamps in a timezone with UT
     offset +01:23:45 (5025 seconds) and with a positive leap second at
     1972-06-30 23:59:60 UTC (78796800):

	time_t    without the fix      with the fix
	78796800  1972-07-01 01:23:45  1972-07-01 01:23:45 (leap second)
	78796801  1972-07-01 01:23:45  1972-07-01 01:23:46
	78796815  1972-07-01 01:23:59  1972-07-01 01:23:60
	78796816  1972-07-01 01:24:00  1972-07-01 01:24:00

     Fix an unlikely bug that caused 'localtime' etc. to misbehave if
     civil time changes a few seconds before time_t wraps around, when
     leap seconds are enabled.

     Fix bug in zic -r; in some cases, the dummy time type after the
     last time transition disagreed with the TZ string, contrary to
     Internet RFC 8563 section 3.3.

     Fix a bug with 'zic -r @X' when X is a negative leap second that
     has a nonnegative correction.  Without the fix, the output file
     was truncated so that X appeared to be a positive leap second.
     Fix a similar, even-less-likely bug when truncating at a positive
     leap second that has a nonpositive correction.

     zic -r now reports an error if given rolling leap seconds, as this
     usage has never generally worked and is evidently unused.

     zic now generates a POSIX-conforming TZ string for TZif files
     where all-year DST is predicted for the indefinite future.
     For example, for all-year Eastern Daylight Time, zic now generates
     "XXX3EDT4,0/0,J365/23" where it previously generated
     "EST5EDT,0/0,J365/25" or "".  (Thanks to Michael Deckers for
     noting the possibility of POSIX conformance.)

     zic.c no longer requires sys/wait.h (thanks to spazmodius for
     noting it wasn't needed).

     When reading slim TZif files, zdump no longer mishandles leap
     seconds on the rare platforms where time_t counts leap seconds,
     fixing a bug introduced in 2014g.

     zdump -v now outputs timestamps at boundaries of what localtime
     and gmtime can represent, instead of the less-useful timestamps
     one day after the minimum and one day before the maximum.
     (Thanks to Arthur David Olson for prototype code, and to Manuela
     Friedrich for debugging help.)

     zdump's -c and -t options are now consistently inclusive for the
     lower time bound and exclusive for the upper.  Formerly they were
     inconsistent.  (Confusion noted by Martin Burnicki.)

   Changes to build procedure

     You can now compile with -DHAVE_MALLOC_ERRNO=0 to port to
     non-POSIX hosts where malloc doesn't set errno.
     (Problem reported by Jan Engelhardt.)

   Changes to documentation

     tzfile.5 better matches a draft successor to RFC 8536

Here are links to the release files:


The following convenience links are also available, although they may
point to the previous release until the relevant caches are refreshed:


Links are also available via plain HTTP, and via FTP from
ftp://ftp.iana.org/tz/releases with the same basenames as above.

Each release file has a GPG signature, which can be retrieved by 
appending ".asc" to the above URLs. Copies of these signatures are 
appended to this message.

This release corresponds to commit 
9ffa3f6e5019dfa7cfb5c0e4e8a84fa95adec704 dated 2021-09-24 16:23:00 -0700 
and tagged '2021b' in the development GitHub repository at 

Here are the SHA-512 checksums for the release files:


Here are the GPG digital signatures for the release files:





PS. If your tzdata parser does not yet support negative DST offsets or 
times past 24:00, or if it insists on a 'pacificnew' file that is no 
longer present, this release's data entries can be turned into a 
rearguard-format tarball that should work even with these older parsers. 
This is intended to be a temporary transition aid for these parsers. To 
generate a rearguard-format tarball, obtain the full distribution as 
described above, and run the command 'make rearguard_tarballs' on a 
development host. Or you can run 'make rearguard.zi' to generate a 
single file that can be fed directly to a parser that works like 'zic'.

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