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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 begin 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, either 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 timezone abbreviation strings stored in the file.

       The  above  header  is  followed by the following fields, whose lengths vary depend on the
       contents of the header:

       * tzh_timecnt four-byte signed integer values sorted in ascending order.  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  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.  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 timezone abbreviation bytes that follow the ttinfo structure(s) in the file.

       *      tzh_leapcnt  pairs  of  four-byte values, written in standard 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 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  ascending  order  by  time.   Each  transition  is  for one leap second, either
              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 timezone 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 timezone file is used  in  handling  POSIX-style
              timezone environment variables.

       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 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  expose  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-maintained timezone data (which is maintained by some other entity).

   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 (with nothing
       between the newlines if there is no POSIX representation for such instants).   The  POSIX-
       style  string must agree with the local time type after both data's last transition times;
       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 daylight-saving time
       abbreviated “WEST” that is one hour east of UT.

   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 4.16 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/.

                                            2017-08-04                                  TZFILE(5)