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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:
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 timezone abbreviation strings stored in the file.
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.
NOTES
This manual page documents <tzfile.h> in the glibc source archive, see timezone/tzfile.h.
It seems that timezone uses tzfile internally, but glibc refuses to expose it to
userspace. This is most likely because the standardised functions are more useful and
portable, and actually documented by glibc. It may only be in glibc just to support the
non-glibc-maintained timezone data (which is maintained by some other entity).
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
This page is part of release 4.15 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
2017-08-04 TZFILE(5)