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: tzset(3)localtime(5)tzselect(8)zdump(8)zic(8)