TZFILE(5)                   BSD File Formats Manual                  TZFILE(5)

NAME
     tzfile -- timezone 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, fol-
     lowed by a character identifying the version of the file's format (as of
     2005, either an ASCII NUL or a '2') followed by fifteen bytes containing
     zeroes reserved for future use, followed by four four-byte 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 UTC/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 values of type
     long, sorted in ascending order.  These values are written in ``stan-
     dard'' byte order.  Each is used as a transition time (as returned by
     time(3)) at which the rules for computing local time change.  Next come
     tzh_timecnt one-byte values of type unsigned char; 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 {
                   long    tt_gmtoff;
                   int     tt_isdst;
                   unsigned int    tt_abbrind;
           };

     Each structure is written as a four-byte value for tt_gmtoff of type
     long, 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 UTC, 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 struc-
     ture(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(3)) 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 UTC/local indicators, each stored as a
     one-byte value; they tell whether the transition times associated with
     local time types were specified as UTC 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 is 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.  After the
     second header and data comes a newline-enclosed, POSIX-TZ-environment-
     variable-style string for use in handling instants after the last transi-
     tion time stored in the file (with nothing between the newlines if there
     is no POSIX representation for such instants).

SEE ALSO
     ctime(3), time2posix(3), zic(8)

BSD                           September 13, 1994                           BSD