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:

         o  The magic four-byte ASCII sequence "TZif" identifies the file as a
            timezone information file.

         o  A  byte  identifying the version of the file's format (as of 2021,
            either an ASCII NUL, "2", "3", or "4").

         o  Fifteen bytes containing zeros reserved for future use.

         o  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:

         o  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.

         o  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 proleptic TZ string
            described  below.)   These  values  serve as indices into the next
            field.

         o  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 entries in the file; if
            the designated string is "-00", the ttinfo entry is a  placeholder
            indicating  that local time is unspecified.  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].

         o  tzh_charcnt bytes that represent time zone designations, which are
            null-terminated byte strings,  each  indexed  by  the  tt_desigidx
            values  mentioned above.  The byte strings can overlap if one is a
            suffix of the  other.   The  encoding  of  these  strings  is  not
            specified.

         o  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 or at which the
            leap  second  table  expires;  the  second  is  a  signed  integer
            specifying  the  correction,  which  is  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 strictly ascending  order
            by  time.   Each  pair denotes one leap second, either positive or
            negative, except that if the last pair has the same correction  as
            the  previous  one,  the last pair denotes the leap second table's
            expiration time.  Each leap second is at the end of a UTC calendar
            month.  The first leap second has a nonnegative  occurrence  time,
            and  is  a  positive  leap second if and only if its correction is
            positive; the correction for each  leap  second  after  the  first
            differs  from  the previous leap second by either 1 for a positive
            leap second, or -1 for a negative leap second.  If the leap second
            table is  empty,  the  leap-second  correction  is  zero  for  all
            timestamps;  otherwise, for timestamps before the first occurrence
            time, the leap-second correction  is  zero  if  the  first  pair's
            correction  is  1  or  -1, and is unspecified otherwise (which can
            happen only in files truncated at the start).

         o  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.

         o  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 proleptic  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 the details of 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 string in the style of the contents of a
       proleptic TZ, 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  TZ  string  is  empty  (i.e.,  nothing  between  the
       newlines)  if  there  is no proleptic representation for such instants.
       If nonempty, the 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,M3.5.0/1,M10.5.0" 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, a TZ string (see newtzset(3))  may
       use the following POSIX.1-2024 extensions to POSIX.1-2017: First, as in
       TZ="<-02>2<-01>,M3.5.0/-1,M10.5.0/0",  the hours part of its transition
       times may be signed and range from -167 through 167  instead  of  being
       limited   to  unsigned  values  from  0  through  24.   Second,  as  in
       TZ="XXX3EDT4,0/0,J365/23", 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 4 format
       For version-4-format TZif files, the first leap second record can  have
       a  correction that is neither +1 nor -1, to represent truncation of the
       TZif file at the start.  Also, if two or more leap  second  transitions
       are  present  and  the last entry's correction equals the previous one,
       the last entry denotes the expiration of the leap second table  instead
       of  a  leap  second; timestamps after this expiration are unreliable in
       that future releases will likely add  leap  second  entries  after  the
       expiration,  and the added leap seconds will change how post-expiration
       timestamps are treated.

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

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

       Other than version 1, writers should generate the lowest version number
       needed by a file's data.  For  example,  a  writer  should  generate  a
       version  4  file  only  if  its  leap second table either expires or is
       truncated at the start.  Likewise, a writer not generating a version  4
       file  should generate a version 3 file only if TZ string extensions are
       necessary to accurately model transition times.

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

       When a TZif file contains a leap second  table  expiration  time,  TZif
       readers  should either refuse to process post-expiration timestamps, or
       process them as if the expiration time did not exist (possibly with  an
       error indication).

       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 higher 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.

       When  a  positive  leap  second  occurs, readers should append an extra
       second to the local minute containing the second just before  the  leap
       second.   If  this  occurs  when the UTC offset is not a multiple of 60
       seconds, the leap second occurs earlier than the  last  second  of  the
       local  minute  and  the  minute's  remaining local seconds are numbered
       through 60 instead of the usual 59; the UTC offset is unaffected.

   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:

         o  to help TZif writers output files that avoid  common  pitfalls  in
            older or buggy TZif readers,

         o  to  help  TZif  readers  avoid  common pitfalls when reading files
            generated by future TZif writers, and

         o  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  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:

         o  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.

         o  Some readers designed for version  2  might  mishandle  timestamps
            after  a  version 3 or higher file's last transition, because they
            cannot parse the POSIX.1-2024 extensions to  POSIX.1-2017  in  the
            proleptic TZ 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.

         o  Some readers designed for  version  2  do  not  support  permanent
            daylight  saving  time  with  transitions after 24:00 - e.g., a TZ
            string "EST5EDT,0/0,J365/25" denoting permanent  Eastern  Daylight
            Time  (-04).   As  a  workaround, a writer can substitute standard
            time for two time zones east, e.g., "XXX3EDT4,0/0,J365/23"  for  a
            time  zone with a never-used standard time (XXX, -03) and negative
            daylight saving time (EDT, -04) all  year.   Alternatively,  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).

         o  Some readers designed for version 2 or 3, and that require  strict
            conformance  to RFC 8536, reject version 4 files whose leap second
            tables are truncated at the start or that end in expiration times.

         o  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.

         o  Some  stripped-down  readers ignore everything but the footer, and
            use its proleptic TZ string to calculate all timestamps.  Although
            this approach often works for current and  future  timestamps,  it
            obviously  has problems with past timestamps, and even for current
            timestamps it can fail for settings  like  TZ="Africa/Casablanca".
            This  corresponds  to  a TZif file containing explicit transitions
            through the year 2087, followed by  a  footer  containing  the  TZ
            string  "<+01>-1",  which should be used only for timestamps after
            the last explicit transition.

         o  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.

         o  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.

         o  Some  readers  mishandle  a  transition  if  its timestamp has the
            minimum possible signed 64-bit value.  Timestamps less than -2**59
            are not recommended.

         o  Some readers mishandle TZ strings that contain "<" or ">".   As  a
            partial  workaround,  a writer can avoid using "<" or ">" for time
            zone abbreviations containing only alphabetic characters.

         o  Many readers mishandle time zone abbreviations that  contain  non-
            ASCII characters.  These characters are not recommended.

         o  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.

         o  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 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  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.

         o  Some readers  generate  ambiguous  timestamps  for  positive  leap
            seconds  that  occur  when  the UTC offset is not a multiple of 60
            seconds.  For example, in a timezone with UTC offset +01:23:45 and
            with a positive leap second 78796801  (1972-06-30  23:59:60  UTC),
            some readers will map both 78796800 and 78796801 to 01:23:45 local
            time  the  next day instead of mapping the latter to 01:23:46, and
            they will map 78796815 to 01:23:59 instead of to  01:23:60.   This
            has not yet been a practical problem, since no civil authority has
            observed  such  UTC  offsets since leap seconds were introduced in
            1972.

       Some interoperability problems are reader bugs  that  are  listed  here
       mostly as warnings to developers of readers.

         o  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.

         o  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.

         o  Some  readers  mishandle  time  zone abbreviations like "-08" that
            contain "+", "-", or digits.

         o  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.

         o  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".

         o  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 doi:10.17487/RFC8536.

Time Zone Database                                                   tzfile(5)
