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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)