tzfile —
time zone information
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 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 standard 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 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_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 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 time (as returned by
          time(3)) 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="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 obsolete
            transformational behavior, the default rules are
            installation-dependent, and no implementation is known to support
            this feature for timestamps past2037, so
            users desiring (say) Greek time should instead specifyTZ="Europe/Athens"for better
            historical coverage, falling back onTZ="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. 
 
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.
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 daylight saving and standard time.
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.
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. Future changes to the format may append
      more data.
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..