TZFILE(5)                     File Formats Manual                    TZFILE(5)

NAME
       tzfile - time zone information

SYNOPSIS
       #include <tzfile.h>

DESCRIPTION
       The time zone information files used by tzset(3) begin with the magic
       characters "TZif" to identify them as time zone information files,
       followed by a character identifying the version of the file's format
       (as of 2013, either an ASCII NUL, or '2', or '3') followed by fifteen
       bytes containing zeroes reserved for future use, followed by 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 is stored in the file.

       tzh_timecnt
              The number of "transition times" for which data is stored in the
              file.

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

       tzh_charcnt
              The number of characters of "time zone abbreviation strings"
              stored in the file.

       The above header is followed by 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.  Next
       come 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 same-indexed transition time.  These values serve
       as indices into an array of ttinfo structures (with tzh_typecnt
       entries) that appears next in the file; these structures are 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 characters that
       follow the ttinfo structure(s) in the file.

       Then there are tzh_leapcnt pairs of four-byte values, written in
       standard byte order; the first value of each pair gives the time (as
       returned by time(2)) at which a leap second occurs; the second gives
       the total number of leap seconds to be applied after the given time.
       The pairs of values are sorted in ascending order by time.

       Then there are 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 time zone file is used in handling POSIX-
       style time zone environment variables.

       Finally there are 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 time zone environment
       variables.

       Localtime 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.

       For version-2-format time zone 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.
       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).

       For version-3-format time zone 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
       newctime(3), newtzset(3), zdump(8), zic(8)

                                                                     TZFILE(5)
