tzfile(5)                     File Formats Manual                    tzfile(5)

NAME
       tzfile - timezone information

DESCRIPTION
       The timezone information files used by tzset(3) are typically found
       under a directory with a name like /usr/share/zoneinfo.  These files
       use the format described in Internet RFC 9636.  Each file is a sequence
       of 8-bit bytes.  In a file, a binary integer is represented by a
       sequence of one or more bytes in network order (bigendian, or high-
       order byte first), with all bits significant, a signed binary integer
       is represented using two's complement, and a boolean is represented by
       a one-byte binary integer that is either 0 (false) or 1 (true).  The
       format begins with a 44-byte header containing the following fields:

         •  The magic four-byte ASCII sequence “TZif” identifies the file as a
            timezone information file.

         •  A byte identifying the version of the file's format (as of 2021,
            either an ASCII NUL, “2”, “3”, or “4”).

         •  Fifteen bytes containing zeros reserved for future use.

         •  Six four-byte integer values, in the following order:

              tzh_ttisutcnt
                The number of UT/local indicators stored in the file.  (UT is
                Universal Time.)

              tzh_ttisstdcnt
                The number of standard/wall indicators stored in the file.

              tzh_leapcnt
                The number of leap seconds for which data entries are stored
                in the file.

              tzh_timecnt
                The number of transition times for which data entries are
                stored in the file.

              tzh_typecnt
                The number of local time types for which data entries are
                stored in the file (must not be zero).

              tzh_charcnt
                The number of bytes of time zone abbreviation strings stored
                in the file.

       The above header is followed by the following fields, whose lengths
       depend on the contents of the header:

         •  tzh_timecnt four-byte signed integer values sorted in ascending
            order.  These values are written in network byte order.  Each is
            used as a transition time (as returned by time(2)) at which the
            rules for computing local time change.

         •  tzh_timecnt one-byte unsigned integer values; each one but the
            last tells which of the different types of local time types
            described in the file is associated with the time period starting
            with the same-indexed transition time and continuing up to but not
            including the next transition time.  (The last time type is
            present only for consistency checking with the proleptic TZ string
            described below.)  These values serve as indices into the next
            field.

         •  tzh_typecnt ttinfo entries, each defined as follows:

              struct ttinfo {
                  int32_t       tt_utoff;
                  unsigned char tt_isdst;
                  unsigned char tt_desigidx;
              };

            Each structure is written as a four-byte signed integer value for
            tt_utoff, in network byte order, followed by a one-byte boolean
            for tt_isdst and a one-byte value for tt_desigidx.  In each
            structure, tt_utoff gives the number of seconds to be added to UT,
            tt_isdst tells whether tm_isdst should be set by localtime(3) and
            tt_desigidx serves as an index into the array of time zone
            abbreviation bytes that follow the ttinfo entries in the file; if
            the designated string is "-00", the ttinfo entry is a placeholder
            indicating that local time is unspecified.  The tt_utoff value is
            never equal to -2**31, to let 32-bit clients negate it without
            overflow.  Also, in realistic applications tt_utoff is in the
            range [-89999, 93599] (i.e., more than -25 hours and less than 26
            hours); this allows easy support by implementations that already
            support the POSIX-required range [-24:59:59, 25:59:59].

         •  tzh_charcnt bytes that represent time zone designations, which are
            null-terminated byte strings, each indexed by the tt_desigidx
            values mentioned above, and each corresponding to a time zone
            abbreviation.  The byte strings can overlap if one is a suffix of
            the other.  The encoding of these strings is not specified.

         •  tzh_leapcnt pairs of four-byte values, written in network byte
            order; the first value of each pair gives the non-negative time
            (as returned by time(2)) at which a leap second occurs or at which
            the leap second table expires; the second is a signed integer
            specifying the correction, which is the total number of leap
            seconds to be applied during the time period starting at the given
            time.  The pairs of values are sorted in strictly ascending order
            by time.  Each pair denotes one leap second, either positive or
            negative, except that if the last pair has the same correction as
            the previous one, the last pair denotes the leap second table's
            expiration time.  Each leap second is at the end of a UTC calendar
            month.  The first leap second has a non-negative occurrence time,
            and is a positive leap second if and only if its correction is
            positive; the correction for each leap second after the first
            differs from the previous leap second by either 1 for a positive
            leap second, or -1 for a negative leap second.  If the leap second
            table is empty, the leap-second correction is zero for all
            timestamps; otherwise, for timestamps before the first occurrence
            time, the leap-second correction is zero if the first pair's
            correction is 1 or -1, and is unspecified otherwise (which can
            happen only in files truncated at the start).

         •  tzh_ttisstdcnt standard/wall indicators, each stored as a one-byte
            boolean; they tell whether the transition times associated with
            local time types were specified as standard time or local (wall
            clock) time.

         •  tzh_ttisutcnt UT/local indicators, each stored as a one-byte
            boolean; they tell whether the transition times associated with
            local time types were specified as UT or local time.  If a
            UT/local indicator is set, the corresponding standard/wall
            indicator must also be set.

       The standard/wall and UT/local indicators were designed for
       transforming a TZif file's transition times into transitions
       appropriate for another time zone specified via a proleptic TZ string
       that lacks rules.  For example, when TZ="EET-2EEST" and there is no
       TZif file "EET-2EEST", the idea was to adapt the transition times from
       a TZif file with the well-known name "posixrules" that was present only
       for this purpose and was a copy of the file "Europe/Brussels", a file
       with a different UT offset.  POSIX does not specify the details of this
       obsolete transformational behavior, the default rules are installation-
       dependent, and no implementation is known to support this feature for
       timestamps past 2037, so users desiring (say) Greek time should instead
       specify TZ="Europe/Athens" for better historical coverage, falling back
       on TZ="EET-2EEST,M3.5.0/3,M10.5.0/4" if conformance to POSIX.1-2017 or
       earlier is required and older timestamps need not be handled
       accurately.

       The localtime(3) function normally uses the first ttinfo structure in
       the file if either tzh_timecnt is zero or the time argument is less
       than the first transition time recorded in the file.

   Version 2 format
       For version-2-format timezone files, the above header and data are
       followed by a second header and data, identical in format except that
       eight bytes are used for each transition time or leap second time.
       (Leap second counts remain four bytes.)  After the second header and
       data comes a newline-enclosed string in the style of the contents of a
       proleptic TZ, for use in handling instants after the last transition
       time stored in the file or for all instants if the file has no
       transitions.  The TZ string is empty (i.e., nothing between the
       newlines) if there is no proleptic representation for such instants.

       If non-empty, the TZ string must agree with the local time type after
       the last transition time if present in the eight-byte data; for
       example, given the string “WET0WEST,M3.5.0/1,M10.5.0” then if a last
       transition time is in July, the transition's local time type must
       specify a daylight-saving time abbreviated “WEST” that is one hour east
       of UT.

       The TZ string can contain time zone abbreviations and UT offsets that
       do not appear elsewhere in the TZif file.

       Also, if there is at least one transition, time type 0 is associated
       with the time period from the indefinite past up to but not including
       the earliest transition time.

   Version 3 format
       For version-3-format timezone files, a TZ string (see newtzset(3)) may
       use the following POSIX.1-2024 extensions to POSIX.1-2017: First, as in
       TZ="<-02>2<-01>,M3.5.0/-1,M10.5.0/0", the hours part of its transition
       times may be signed and range from -167 through 167 instead of being
       limited to unsigned values from 0 through 24.  Second, as in
       TZ="XXX3EDT4,0/0,J365/23", DST is in effect all year if it starts
       January 1 at 00:00 and ends December 31 at 24:00 plus the difference
       between daylight saving and standard time.

   Version 4 format
       For version-4-format TZif files, the first leap second record can have
       a correction that is neither +1 nor -1, to represent truncation of the
       TZif file at the start.  Also, if two or more leap second transitions
       are present and the last entry's correction equals the previous one,
       the last entry denotes the expiration of the leap second table instead
       of a leap second; timestamps after this expiration are unreliable in
       that future releases will likely add leap second entries after the
       expiration, and the added leap seconds will change how post-expiration
       timestamps are treated.

   Interoperability considerations
       Future changes to the format may append more data.

       Version 1 files are considered a legacy format and should not be
       generated, as they do not support transition times after the year 2038.
       Readers that understand only Version 1 must ignore any data that
       extends beyond the calculated end of the version 1 data block.

       Other than version 1, writers should generate the lowest version number
       needed by a file's data.  For example, a writer should generate a
       version 4 file only if its leap second table either expires or is
       truncated at the start.  Likewise, a writer not generating a version 4
       file should generate a version 3 file only if TZ string extensions are
       necessary to accurately model transition times.

       The sequence of time changes defined by the version 1 header and data
       block should be a contiguous sub-sequence of the time changes defined
       by the version 2+ header and data block, and by the footer.  This
       guideline helps obsolescent version 1 readers agree with current
       readers about timestamps within the contiguous sub-sequence.  It also
       lets writers not supporting obsolescent readers use a tzh_timecnt of
       zero in the version 1 data block to save space.

       When a TZif file contains a leap second table expiration time, TZif
       readers should either refuse to process post-expiration timestamps, or
       process them as if the expiration time did not exist (possibly with an
       error indication).

       Time zone abbreviations should consist of at least three (3) and no
       more than six (6) ASCII characters from the set of alphanumerics, “-”,
       and “+”.  This is for compatibility with POSIX requirements for time
       zone abbreviations.

       A numeric time zone abbreviation should match the UT offset.  For
       example, "+0530" should be used only if the UT offset is 5.5 hours
       ahead of UT, and "-00" should be used only if the UT offset is zero.

       When reading a version 2 or higher file, readers should ignore the
       version 1 header and data block except for the purpose of skipping over
       them.

       Readers should calculate the total lengths of the headers and data
       blocks and check that they all fit within the actual file size, as part
       of a validity check for the file.

       When a positive leap second occurs, readers should append an extra
       second to the local minute containing the second just before the leap
       second.  If this occurs when the UTC offset is not a multiple of 60
       seconds, the leap second occurs earlier than the last second of the
       local minute and the minute's remaining local seconds are numbered
       through 60 instead of the usual 59; the UTC offset is unaffected.

   Common interoperability issues
       This section documents common problems in reading or writing TZif
       files.  Most of these are problems in generating TZif files for use by
       older readers.  The goals of this section are to help:

         •  TZif writers output files that avoid common pitfalls in older or
            buggy TZif readers,

         •  TZif readers avoid common pitfalls when reading files generated by
            future TZif writers, and

         •  any future specification authors see what sort of problems arise
            when the TZif format is changed.

       When new versions of the TZif format have been defined, a design goal
       has been that a reader can successfully use a TZif file even if the
       file is of a later TZif version than what the reader was designed for.
       When complete compatibility was not achieved, an attempt was made to
       limit glitches to rarely used timestamps and allow simple partial
       workarounds in writers designed to generate newer-version data useful
       even for older-version readers.  This section attempts to document
       these compatibility issues and workarounds as well as documenting other
       common bugs in readers.

       Interoperability problems with TZif include the following:

         •  Some readers examine only version 1 data.  As a partial
            workaround, a writer can output as much version 1 data as
            possible.  However, a reader should ignore version 1 data, and
            should use version 2+ data even if the reader's native timestamps
            have only 32 bits.

         •  Some readers designed for version 2 might mishandle timestamps
            after a version 3 or higher file's last transition, because they
            cannot parse the POSIX.1-2024 extensions to POSIX.1-2017 in the
            proleptic TZ string.  As a partial workaround, a writer can output
            more transitions than necessary, so that only far-future
            timestamps are mishandled by version 2 readers.

         •  Some readers might mishandle timestamps after a file's last
            transition, because they require that all abbreviations or UT
            offsets in the proleptic TZ string must also occur somewhere in
            the file's tables of time zone designations and local time type
            records.  As a workaround, a writer can output more transitions
            than necessary, so that the other tables contain duplicates of the
            proleptic TZ string's abbreviations and offsets.

         •  Some readers designed for version 2 do not support permanent
            daylight saving time with transitions after 24:00 – e.g., a TZ
            string “EST5EDT,0/0,J365/25” denoting permanent Eastern Daylight
            Time (-04).  As a workaround, a writer can substitute standard
            time for two time zones east, e.g., “XXX3EDT4,0/0,J365/23” for a
            time zone with a never-used standard time (XXX, -03) and negative
            daylight saving time (EDT, -04) all year.  Alternatively, as a
            partial workaround, a writer can substitute standard time for the
            next time zone east – e.g., “AST4” for permanent Atlantic Standard
            Time (-04).

         •  Some readers designed for version 2 or 3 and that require strict
            conformance to RFC 9636 reject version 4 files whose leap second
            tables are truncated at the start or end in expiration times.

         •  Some readers ignore the footer, and instead predict future
            timestamps from the time type of the last transition.  As a
            partial workaround, a writer can output more transitions than
            necessary.

         •  Some stripped-down readers ignore everything but the footer, and
            use its proleptic TZ string to calculate all timestamps.  Although
            this approach often works for current and future timestamps, it
            obviously has problems with past timestamps, and even for current
            timestamps it can fail for settings like TZ="Africa/Casablanca".
            This corresponds to a TZif file containing explicit transitions
            through the year 2087, followed by a footer containing the TZ
            string “<+01>-1”, which should be used only for timestamps after
            the last explicit transition.

         •  Some readers do not use time type 0 for timestamps before the
            first transition, in that they infer a time type using a heuristic
            that does not always select time type 0.  As a partial workaround,
            a writer can output a dummy (no-op) first transition at an early
            time.

         •  Some readers mishandle timestamps before the first transition that
            has a timestamp that is not less than -2**31.  Readers that
            support only 32-bit timestamps are likely to be more prone to this
            problem, for example, when they process 64-bit transitions only
            some of which are representable in 32 bits.  As a partial
            workaround, a writer can output a dummy transition at timestamp
            -2**31.

         •  Some readers mishandle a transition if its timestamp has the
            minimum possible signed 64-bit value.  Timestamps less than -2**59
            are not recommended.

         •  Some readers mishandle proleptic TZ strings that contain “<” or
            “>”.  As a partial workaround, a writer can avoid using “<” or “>”
            for time zone abbreviations containing only alphabetic characters.

         •  Many readers mishandle time zone abbreviations that contain non-
            ASCII characters.  These characters are not recommended.

         •  Some readers may mishandle time zone abbreviations that contain
            fewer than 3 or more than 6 characters or that contain ASCII
            characters other than alphanumerics, “-”, and “+”.  These
            abbreviations are not recommended.

         •  Some readers mishandle TZif files that specify daylight-saving
            time UT offsets that are less than the UT offsets for the
            corresponding standard time.  These readers do not support
            locations like Ireland, which uses the equivalent of the TZ string
            “IST-1GMT0,M10.5.0,M3.5.0/1”, observing standard time (IST, +01)
            in summer and daylight saving time (GMT, +00) in winter.  As a
            partial workaround, a writer can output data for the equivalent of
            the TZ string “GMT0IST,M3.5.0/1,M10.5.0”, thus swapping standard
            and daylight saving time.  Although this workaround misidentifies
            which part of the year uses daylight saving time, it records UT
            offsets and time zone abbreviations correctly.

         •  Some readers generate ambiguous timestamps for positive leap
            seconds that occur when the UTC offset is not a multiple of 60
            seconds.  For example, with UTC offset +01:23:45 and a positive
            leap second 78796801 (1972-06-30 23:59:60 UTC), some readers will
            map both 78796800 and 78796801 to 01:23:45 local time the next day
            instead of mapping the latter to 01:23:46, and they will map
            78796815 to 01:23:59 instead of to 01:23:60.  This has not yet
            been a practical problem, since no civil authority has observed
            such UTC offsets since leap seconds were introduced in 1972.

       Some interoperability problems are reader bugs that are listed here
       mostly as warnings to developers of readers.

         •  Some readers do not support negative timestamps.  Developers of
            distributed applications should keep this in mind if they need to
            deal with pre-1970 data.

         •  Some readers mishandle timestamps before the first transition that
            has a non-negative timestamp.  Readers that do not support
            negative timestamps are likely to be more prone to this problem.

         •  Some readers mishandle time zone abbreviations like “-08” that
            contain “+”, “-”, or digits.

         •  Some readers mishandle UT offsets that are out of the traditional
            range of -12 through +12 hours, and so do not support locations
            like Kiritimati that are outside this range.

         •  Some readers mishandle UT offsets in the range [-3599, -1] seconds
            from UT because they integer-divide the offset by 3600 to get 0
            and then display the hour part as “+00”.

         •  Some readers mishandle UT offsets that are not a multiple of one
            hour, or of 15 minutes, or of 1 minute.

SEE ALSO
       time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8).

       Olson A, Eggert P, Murchison K. The Time Zone Information Format
       (TZif).  October 2024.  Internet RFC 9636 doi:10.17487/RFC9636.

Time Zone Database                                                   tzfile(5)
