NAME
tzfile —
time zone information
DESCRIPTION
The time zone information files used by
tzset(3) begin with the magic
characters “TZif” to identify them as time zone information files,
followed by a character identifying the version of the file's format (as of
2013, either an ASCII NUL or a '2', or '3') followed by fifteen bytes
containing zeroes reserved for future use, followed by six four-byte values of
type
long, followed by 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 characters of time zone abbreviation strings
stored in the file.
The above header is followed by
tzh_timecnt four-byte
signed integer values sorted in ascending order. These values are written in
These values are written in standard byte order. Each is used as a transition
time (as returned by
time(3)) at
which the rules for computing local time change. Next come
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 an array of
ttinfo structures (with
tzh_typecnt entries) that appears next in the file;
these structures are 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 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(3)) 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.
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 time zone file is used in handling POSIX-style time
zone environment variables.
Finally there are
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
time zone file is used in handling POSIX-style time zone 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.
For version-2-format time zone 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. 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 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.
For version-3-format time zone files, the POSIX-TZ-style string may use two
minor extensions to the POSIX TZ format, as described in
tzset(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
ctime(3),
localtime(3),
time(3),
tzset(3),
zdump(8)
zic(8)