bsd man page for tzfile

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 bytes reserved for future use, followed by three four-byte values of type long,
       written in a ``standard'' byte order (the high-order byte of the value is written first).  These values are, in order:

       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(2)) 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 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 GMT, 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.

       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.

SEE ALSO
       ctime(3)

System V																 TZFILE(5)