tzfile

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  be-
       gin 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 written in a standard byte order (the
         high-order byte of the value is written first).  These values are, in
         order:

         tzh_ttisgmtcnt
                The number of UT/local indicators stored in the file.

         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 standard 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_gmtoff;
                   unsigned char  tt_isdst;
                   unsigned char  tt_abbrind;

              };

         Each  structure  is  written  as a four-byte signed integer value for
         tt_gmtoff, in a standard byte order, followed by a one-byte value for
         tt_isdst  and  a  one-byte  value for tt_abbrind.  In each structure,
         tt_gmtoff gives the number of seconds to be  added  to  UT,  tt_isdst
         tells  whether  tm_isdst should be set by localtime(3) and tt_abbrind
         serves as an index into the array of  time  zone  abbreviation  bytes
         that follow the ttinfo structure(s) in the file.

       * tzh_leapcnt  pairs  of four-byte values, written in standard byte or-
         der; the first value of each pair gives the nonnegative time (as  re-
         turned  by  time(2))  at which a leap second occurs; the second gives
         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 as-
         cending order by time.  Each transition is for one leap  second,  ei-
         ther  positive  or negative; transitions always separated by at least
         28 days minus 1 second.

       * tzh_ttisstdcnt standard/wall indicators, each stored  as  a  one-byte
         value;  they  tell whether the transition times associated with local
         time types were specified as standard time or wall  clock  time,  and
         are  used  when a timezone file is used in handling POSIX-style time-
         zone environment variables.

       * tzh_ttisgmtcnt UT/local indicators, each stored as a one-byte  value;
         they  tell  whether  the  transition times associated with local time
         types were specified as UT or local time, and are used when  a  time-
         zone  file is used in handling POSIX-style timezone environment vari-
         ables.

       The localtime(3) function uses the first standard-time ttinfo structure
       in  the  file (or simply the first ttinfo structure in the absence of a
       standard-time structure) if either tzh_timecnt is zero or the time  ar-
       gument 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.

       Future changes to the format may append more data.

SEE ALSO
       time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8)

COLOPHON
       This page is part of release 5.05 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

                                  2019-03-06                         TZFILE(5)