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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 tran-
       sition 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 sec-
            ond  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 second, 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  han-
            dling 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  time-
            zone environment 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.

 Version 2 format
     For version-2-format timezone files, the above header and data are 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.
     (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.