Ubuntu Manpage: tzfile - timezone information

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 "standard" 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 standard 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 environment 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.

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 is followed 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 timezone 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

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                                                   2015-05-07                                          TZFILE(5)

NAME

DESCRIPTION

NOTES

SEE ALSO

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