xref: /third_party/tzdata/tzfile.5.txt

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.1-2017-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.1-2017-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 string in the style of the contents of  a
       POSIX.1-2017  TZ  environment  variable,  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 POSIX.1-2017-style  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, the TZ string may use two minor
       extensions to the POSIX.1-2017 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.1-2017 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 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)