TZFILE(5)                 NetBSD File Formats Manual                 TZFILE(5)

NAME
     tzfile -- time zone 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 inte-
     ger that is either 0 (false) or 1 (true).

        The magic four-byte ASCII sequence begin with the magic characters
         ``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.

         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 timezone 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 These values are written in stan-
         dard byte order.  Each is used as a transition time (as returned by
         time(3)) 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 con-
         sistency 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_uttoff;
                 unsigned char   tt_isdst;
                 unsigned char   tt_desigind;
         };

         Each structure is written as a four-byte signed integer value for
         tt_gmtoff in a network byte order, followed by a one-byte value for
         tt_isdst and a one-byte value for tt_desigidx.  In each structure,
         tt_gmtoff 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 timezone 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 over-
         flow.  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 time (as returned by time(3))
         at which a leap second occurs; the second is a signed integer speci-
         fying 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 trans-
         forming 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="EET2EEST" and there is no TZif
         file ``EET2EEST'', 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 obso-
         lete 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="EET2EEST,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 fol-
     lowed by a second header and data, identical in format except that eight
     bytes are used for each transition time or leap second time.  (Leap sec-
     ond 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 tzset(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 day-
     light saving and standard time.

   Interoperability considerations
     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 gener-
         ated 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 work-
     arounds in writers designed to generate new-version data useful even for
     older-version readers.  This section attempts to document these compati-
     bility 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 exten-
         sions 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 per-
         manent 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 out-
         put 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 charac-
         ters.

         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 dis-
         tributed 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.  Future changes to the format
         may append more data.

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

     Olson A, Eggert P, Murchison K., The Time Zone Information Format
     (TZif)., RFC 8536, https://www.rfc-editor.org/info/rfc8536
     https://doi.org/10.17487/RFC8536, Feb 2019..

NetBSD 9.0                       July 2, 2019                       NetBSD 9.0

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