NAME
tzfile —
time zone information
DESCRIPTION
The time zone information files used by
tzset(3) are typically found
under a directory with a name like
/usr/share/zoneinfo.
These files beging with a 44-byte header containing the following fields:
- The magic four-byte ASCII sequence begin with the magic
characters “TZif”. identifies the file as a time zone
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 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.
- 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
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 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.
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 time zone file is used in handling POSIX-style time zone
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 time
zone file is used in handling POSIX-style time zone 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 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. (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.
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)