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  POSIX-
              style 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  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-style  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,M3.5.0/1,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, 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.

   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 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.

         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 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 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.

         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 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.

         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)
