tzfile(5) — Linux manual page

NAME | DESCRIPTION | SEE ALSO | COLOPHON

TZFILE(5)                 Linux Programmer's 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 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         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://www.rfc-editor.org/info/rfc8536⟩ doi:10.17487/RFC8536 
       ⟨https://doi.org/10.17487/RFC8536⟩.

COLOPHON         top

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                                 2020-04-27                        TZFILE(5)

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