tzfile(5) — Linux manual page

NAME | DESCRIPTION | SEE ALSO | COLOPHON

tzfile(5)                  File Formats Manual                 tzfile(5)

NAME         top

       tzfile - timezone information

DESCRIPTION         top

       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 8536.  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 POSIX.1-2017-style 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.  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
            nonnegative 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 nonnegative
            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
       POSIX.1-2017-style 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 is present only for this purpose and
       is a copy of the file "Europe/Brussels", a file with a different
       UT offset.  POSIX does not specify 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 POSIX
       conformance 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 POSIX.1-2017 TZ environment variable,
       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 POSIX.1-2017-style representation for
       such instants.  If nonempty, 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.
       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, the TZ string may use two
       minor extensions to the POSIX.1-2017 TZ format, as described in
       newtzset(3).  First, the hours part of its transition times may
       be signed and range from -167 through 167 instead of the POSIX-
       required unsigned values from 0 through 24.  Second, 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 designations 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.

       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,

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

         •  to help 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
       new-version data useful even for older-version readers.  This
       section attempts to document these compatibility issues and
       workarounds, as well as to document 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 extensions to
            POSIX.1-2017 in the TZ-like 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 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 8536, reject version 4 files whose
            leap second tables are truncated at the start or that 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 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 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 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, in a timezone with UTC offset
            +01:23:45 and with 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 nonnegative 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         top

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

       Olson A, Eggert P, Murchison K. The Time Zone Information Format
       (TZif).  2019 Feb.  Internet RFC 8536 
       ⟨https://datatracker.ietf.org/doc/html/rfc8536⟩
       doi:10.17487/RFC8536 ⟨https://doi.org/10.17487/RFC8536⟩.

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Time Zone Database                                             tzfile(5)

Pages that refer to this page: tzset(3)localtime(5)tzselect(8)zdump(8)zic(8)