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 9636.  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  non-negative  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 non-negative 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  non-empty,  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 to help:

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

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

         o  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 newer-version  data  useful
       even  for  older-version  readers.   This  section attempts to document
       these compatibility issues and workarounds as well as documenting 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 9636 reject version 4 files whose leap second
            tables are truncated at the start or 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 that  is  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  proleptic 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, with UTC offset +01:23:45  and  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  non-negative  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).  October 2024.  Internet RFC 9636 doi:10.17487/RFC9636.

Time Zone Database                                                   tzfile(5)
