tzfile

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 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 2017, either
         an ASCII NUL, or “2”, or “3”).

       * 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-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 structure(s) in the file.  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_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; the second is a signed integer
         specifying 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 ascending order by time.  Each transition is for one leap second,
         either positive or negative; transitions always separated by at least
         28 days minus 1 second.

       * 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-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, POSIX-TZ-environment-variable-style string 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 POSIX-style TZ
       string is empty (i.e., nothing between the newlines) if there is no POSIX
       representation for such instants.  If nonempty, the POSIX-style 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,M10.5.0/3” 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 POSIX-TZ-style string may use
       two minor extensions to the POSIX 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.

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

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

       Writers should generate a version 3 file if TZ string extensions are
       necessary to accurately model transition times.  Otherwise, version 2
       files should be generated.

       The sequence of time changes defined by the version 1 header and data
       block should be a contiguous subsequence 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 subsequence.  It also lets writers not
       supporting obsolescent readers use a tzh_timecnt of zero in the version 1
       data block to save space.

       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 3 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.

   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 to 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 file's last transition, because they cannot parse extensions
         to POSIX 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, e.g., a TZ string “EST5EDT,0/0,J365/25” denoting permanent
         Eastern Daylight Time (-04).  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 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 POSIX-style 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 POSIX 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 POSIX 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 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
       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://www.rfc-editor.org/info/rfc8536⟩
       doi:10.17487/RFC8536 ⟨https://doi.org/10.17487/RFC8536⟩.



                                                                       TZFILE(5)