tzfile(5)



TZFILE(5)                  Linux Programmer's Manual                 TZFILE(5)

NAME
       tzfile - timezone information

DESCRIPTION
       The timezone information files used by tzset(3) are typically found un-
       der 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  repre-
       sented  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:

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

       * A byte identifying the version of the file's format (as of 2017,  ei-
         ther 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.  (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 de-
       pend on the contents of the header:

       * tzh_timecnt  four-byte  signed integer values sorted in ascending or-
         der.  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.

       * 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  con-
         sistency  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_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 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 nonnegative time (as  returned
         by time(2)) at which a leap second occurs; the second is a signed in-
         teger specifying the total number of leap seconds to be applied  dur-
         ing  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 sepa-
         rated 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  transform-
       ing a TZif file's transition times into transitions appropriate for an-
       other 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 behav-
       ior, the default rules are installation-dependent, and  no  implementa-
       tion  is  known  to  support  this feature for timestamps past 2037, so
       users  desiring  (say)  Greek  time  should  instead  specify   TZ="Eu-
       rope/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.

NOTES
       This  manual page documents <tzfile.h> in the glibc source archive, see
       timezone/tzfile.h.

       It seems that timezone uses tzfile internally, but glibc refuses to ex-
       pose  it  to  userspace.   This is most likely because the standardised
       functions are more useful and  portable,  and  actually  documented  by
       glibc.   It  may  only  be in glibc just to support the non-glibc-main-
       tained timezone data (which is maintained by some other entity).

   Version 2 format
       For version-2-format timezone files, the above header and data are fol-
       lowed  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  day-
       light-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.

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

       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 guide-
       line helps obsolescent version 1 readers  agree  with  current  readers
       about timestamps within the contiguous subsequence.  It also lets writ-
       ers 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.

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

       * to help TZif readers avoid common pitfalls when reading files  gener-
         ated 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  exten-
         sions  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  perma-
         nent  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 out-
         put 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 day-
         light 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  dis-
         tributed  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  con-
         tain "+", "-", 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.

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 <https://www.rfc-editor.org/info/
       rfc8536> doi:10.17487/RFC8536 <https://doi.org/10.17487/RFC8536>.

COLOPHON
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       https://www.kernel.org/doc/man-pages/.

                                  2020-04-27                         TZFILE(5)

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