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



NAME
       tzfile - timezone information

DESCRIPTION
       This  page  describes  the  structure  of  the  timezone	 files used by
       tzset(3).  These files are typically found under one of the directories
       /usr/lib/zoneinfo or /usr/share/zoneinfo.

       Timezone	 information  files  begin with a 44-byte header structured as
       follows:

       *  The magic four-byte sequence "TZif" identifying this as  a  timezone
	  information file.

       *  A  single  character	identifying  the version of the file's format:
	  either an ASCII NUL ('\0') or a '2' (0x32).

       *  Fifteen bytes containing zeros reserved for future use.

       *  Six 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_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 "timezone 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  (with tzh_typecnt
       entries) that appear 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 timezone  abbreviation  characters
       that follow the ttinfo structure(s) in the file.

       Then  there are tzh_leapcnt pairs of four-byte values, written in stan-
       dard 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 timezone file is used in handling POSIX-style
       timezone 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 timezone file is used in handling POSIX-style timezone  environ-
       ment variables.

       localtime(3)  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.

   Version 2 format
       For version-2-format timezone 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  (and
       that  the  version byte in the header record is 0x32 rather than 0x00).
       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  second  section  of	 the timezone file consists of another 44-byte
       header record, identical in structure to the one at  the	 beginning  of
       the  file,  except  that	 it applies to the data that follows, which is
       also identical in structure to the first section of the timezone	 file,
       with the following differences:

       *  The transition time values, after the header, are eight-byte values.

       *  In  each  leap second record, the leap second value is an eight-byte
	  value.  The accumulated leap	second	count  is  still  a  four-byte
	  value.

       In  all	cases,	the eight-byte time values are given in the "standard"
       byte order, the high-order byte first.

   POSIX timezone string
       The second eight-byte time value section is  followed  by  an  optional
       third  section:	a  single  ASCII newline character ('\n'), then a text
       string followed by a second newline character.  The text	 string	 is  a
       POSIX timezone string, whose format is described in the tzset(3) manual
       page.

       The POSIX timezone string defines a rule for computing transition times
       that  follow the last transition time explicitly specified in the time-
       zone information file.

   Summary of the timezone information file format

	      Four-byte value section
	      (header version 0x00 or 0x32)
		      Header record
		      Four-byte transition times
		      Transition time index
		      ttinfo structures
		      Timezone abbreviation array
		      Leap second records
		      Standard/Wall array
		      UTC/Local array

	      Eight-byte value section
	      (only if first header version is 0x32,
	      the second header's version is also 0x32)
		      Header record
		      Eight-byte transition times
		      Transition time index
		      ttinfo structures
		      Timezone abbreviation array
		      Leap second records
		      Standard/Wall array
		      UTC/Local array

	      Third section
	      (optional, only in 0x32 version files)
		      Newline character
		      Timezone string
		      Newline character

SEE ALSO
       ctime(3), tzset(3), tzselect(8),

       timezone/tzfile.h in the glibc source tree

COLOPHON
       This page is part of release 4.10 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/.



				  2015-05-07			     TZFILE(5)