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

NAME         top

       tzfile - timezone information

DESCRIPTION         top

       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:

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

                The number of standard/wall indicators stored in the file.

                The number of leap seconds for which data entries are stored
                in the file.

                The number of transition times for which data entries are
                stored in the file.

                The number of local time types for which data entries are
                stored in the file (must not be zero).

                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

       * 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 han‐
              dling 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 struc‐
       ture 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

   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         top

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

COLOPHON         top

       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

                                 2017-08-04                        TZFILE(5)

Pages that refer to this page: tzset(3)localtime(5)tzselect(8)zdump(8)zic(8)