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

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 sec-
	      ond, 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  environ-
	      ment 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 4.15 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)