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:

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 transition 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 second 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 handling 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 timezone 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.

SEE ALSO¶

time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8)

COLOPHON¶

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