tzfile - timezone information
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 time zone abbreviation strings stored in the
file.
The above header is followed by the following fields, whose lengths 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 but the last
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 and continuing up to but not including the next transition
time. (The last time type is present only for consistency checking with
the POSIX-style TZ string described below.) 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 time zone 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.
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 or for all instants if the file
has no transitions. The POSIX-style TZ string is empty (i.e., nothing between
the newlines) if there is no POSIX representation for such instants. If
nonempty, the POSIX-style TZ string must agree with the local time type after
the last transition time if present in the eight-byte data; 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. Also, if there is at least one transition, time type 0 is associated with
the time period from the indefinite past up to but not including the earliest
transition time.
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.
time(2),
localtime(3),
tzset(3),
tzselect(8),
zdump(8),
zic(8)