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

COLOPHON

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