tzfile man page

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 above header is followed by the following fields, whose lengths vary depend on the contents of the header:

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.

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 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

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Referenced By

localtime(5), tzselect(8), tzset(3), zdump(8), zic(8).

2017-08-04 Linux Programmer's Manual