The Time Zone Database (often called tz, tzdb, or zoneinfo) contains
data that represents the history of local time for many locations around
the world, and supports conversion of UTC time to local time at those
locations to allow display of those local times.
It is updated periodically to reflect changes made by political bodies to
summer daylight saving time rules, UTC offsets, and time zone boundaries.
Three data packages are available: base 'tzdata' is always installed;
optional 'tzdata-right' provides TAI-10s time in the 'right' subtree;
and optional 'tzdata-posix' provides the same zones and times as base
data in the 'posix' subtree, as an explicit distinction from 'right'.
To enable newlib CLOCK_TAI support, the base tzdata package now installs:
/usr/share/zoneinfo/leap-seconds.list
with a symbolic link to the standard location:
/etc/leap-seconds.list.
The tzcode package provides the tzselect, zdump, and zic utilities.
For more information, see the project home page:
https://www.iana.org/time-zones
The following packages have been upgraded in the Cygwin distribution:
- tzcode 2025c utilities
- tzdata 2025c base zones tree
- tzdata-posix 2025c posix zones subtree
- tzdata-right 2025c TAI-10s right zones subtree
For more details on changes, see the announcement or below:
https://lists.iana.org/hyperkitty/list/[email protected]/2025/12/
2025-12-10 2025c
Briefly: Several code changes for compatibility with FreeBSD.
Changes to past timestamps
Baja California agreed with Californiaâs DST rules in 1953 and in 1961
through 1975, instead of observing standard time all year.
Changes to commentary
The leapseconds file contains commentary about the IERS and NIST
last-modified and expiration timestamps for leap second data.
Commentary now also uses characters from the set ââââââ¢â¤ as this
can be
useful and should work with current applications.
This also affects data in iso3166.tab and zone1970.tab, which now
contain strings like âCôte dâIvoireâ instead of âCôte d'Ivoireâ.
Changes to build procedure
Files in distributed tarballs now have correct commit times.
Formerly, the committerâs time zone was incorrectly ignored.
Distribution products (*.asc, *.gz, and *.lz) now have reproducible
timestamps.
Formerly, only the contents of the compressed tarballs had reproducible
timestamps.
By default, distributed formatted man pages (*.txt) now use UTF-8 and
are left-adjusted more consistently.
A new Makefile macro MANFLAGS can override these defaults.
Changes to code
An unset TZ is no longer invalid when /etc/localtime is missing, and is
abbreviated "UTC" not "-00". This reverts to 2024b behavior.
New function offtime_r, short for fixed-offset localtime_rz.
It is defined if STD_INSPIRED is defined.
tzset etc. are now more cautious about questionable TZ settings.
Privileged programs now reject TZ settings that start with '/', unless
they are TZDEFAULT (default "/etc/localtime") or start with TZDIR then
'/' (default "/usr/share/zoneinfo/").
Unprivileged programs now require files to be regular files and reject
relative names containing ".." directory components; formerly, only
privileged programs did those two things.
These changes were inspired by similar behavior in FreeBSD.
On NetBSD, unprivileged programs now use O_REGULAR to check whether a TZ
setting starting with '/' names a regular file, avoiding a minor
security race still present elsewhere.
TZ strings taken from tzalloc arguments are now treated with no less
caution than TZ strings taken from the environment, as the old
undocumented behavior would have been hard to explain.
tzset etc. no longer use the âaccessâ system call to check access;
instead they now use the system calls issetugid, getauxval,
getresuid/getresgid, and geteuid/getegid/getuid/getgid (whichever first
works) to test whether a program is privileged.
Compile with -DHAVE_SYS_AUXV_H=[01] to enable or disable <sys/auxv.h>
which (if it defines AT_SECURE) enables getauxval, and compile with
-DHAVE_ISSETUGID=[01], -DHAVE_GETRESUID=[01], and -DHAVE_GETEUID=[01] to
enable or disable the other callsâ use.
The new CFLAGS option -DTZ_CHANGE_INTERVAL=N makes tzset etc. check for
TZif file changes if the in-memory data are N seconds old or more, and
are derived from the TZ environment variable.
This is intended for platforms that want tzset etc. to reflect changes
to whatever file TZ selects (including changes to /etc/localtime if TZ
is unset).
If N is negative (the default) these checks are omitted; this is the
traditional behavior.
The new CFLAGS options -DHAVE_STRUCT_STAT_ST_CTIM=0 and
-DHAVE_STRUCT_TIMESPEC=0 port to non-POSIX.1-2008 platforms that lack
st_ctim and struct timespec, respectively.
tzset etc. now treat ' ' like '_' in time zone abbreviations, just as
they treat other invalid bytes.
This continues the transition begun in release 96k, which removed spaces
in tzdata because the spaces break time string parsers.
The new CFLAGS option -DTHREAD_PREFER_SINGLE causes tzcode in
single-threaded processes to avoid locks, as FreeBSD does.
This can save time in single-threaded apps.
The threadedness testing costs CPU time and energy in multi-threaded apps.
New options -DHAVE___ISTHREADED and -DHAVE_SYS_SINGLE_THREADED_H can
help configure how to test for single-threadedness.
The new CFLAGS option -DTHREAD_RWLOCK uses read-write locks, as macOS
does, instead of mutexes.
This saves real time when TZ is rarely changing and many threads call
tzcode simultaneously.
It costs more CPU time and energy.
The new CFLAGS option -TTHREAD_TM_MULTI causes localtime to return a
pointer to thread-specific memory, as FreeBSD does, instead of to the
same memory in all threads.
This supports unportable programs that incorrectly use localtime instead
of localtime_r.
This option affects gmtime and offtime similarly to localtime.
Because the corresponding storage is freed on thread exit, this option
is incompatible with POSIX.1-2024 and earlier.
It also costs CPU time and memory.
tzfree now preserves errno, consistently with POSIX.1-2024 âfreeâ.
tzcode now uses mempcpy if available, guessing its availability.
Compile with -DHAVE_MEMPCPY=1 or 0 to override the guess.
tzcode now uses strnlen to improve asymptotic performance a bit.
Compile with -DHAVE_STRNLEN=0 if your platform lacks it.
tzcode now hand-declares unistd.h-provided symbols like getopt if
HAVE_UNISTD_H=0, not if HAVE_POSIX_DECLS=0.
tzset etc. now have an experimental OPENAT_TZDIR option; see Makefile
and localtime.c for details.
On platforms like GNU/Hurd that do not define PATH_MAX, exceedingly long
TZ strings no longer fail merely because they exceed an arbitrary file
name length limit imposed by tzcode.
zic has new options inspired by FreeBSD.
â-Dâ skips creation of output ancestor directories, â-m MODEâ sets
output filesâ mode, and â-u OWNER[:GROUP]â sets output filesâ owner and
group.
zic now uses the fdopen function, which was standardized by POSIX.1-1988
and is now safe to use in portable code.
This replaces its use of the older umask function, which complicated
maintenance.
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