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TZFILE(5) Linux Programmer's Manual TZFILE(5)
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 use the format described in Internet RFC
8536. Each file is a sequence of 8-bit bytes. In a file, a binary integer is represented
by a sequence of one or more bytes in network order (bigendian, or high-order byte first),
with all bits significant, a signed binary integer is represented using two's complement,
and a boolean is represented by a one-byte binary integer that is either 0 (false) or 1
(true). The format begins 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, in the following order:
tzh_ttisutcnt
The number of UT/local indicators stored in the file. (UT is Universal Time.)
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 network 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 in-
cluding 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_utoff;
unsigned char tt_isdst;
unsigned char tt_desigidx;
};
Each structure is written as a four-byte signed integer value for tt_utoff, in network
byte order, followed by a one-byte boolean for tt_isdst and a one-byte value for tt_de-
sigidx. In each structure, tt_utoff gives the number of seconds to be added to UT,
tt_isdst tells whether tm_isdst should be set by localtime(3) and tt_desigidx serves as
an index into the array of time zone abbreviation bytes that follow the ttinfo struc-
ture(s) in the file. The tt_utoff value is never equal to -2**31, to let 32-bit clients
negate it without overflow. Also, in realistic applications tt_utoff is in the range
[-89999, 93599] (i.e., more than -25 hours and less than 26 hours); this allows easy
support by implementations that already support the POSIX-required range [-24:59:59,
25:59:59].
* tzh_leapcnt pairs of four-byte values, written in network byte order; the first value of
each pair gives the nonnegative time (as returned by time(2)) at which a leap second oc-
curs; the second is a signed integer specifying 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 posi-
tive or negative; transitions always separated by at least 28 days minus 1 second.
* tzh_ttisstdcnt standard/wall indicators, each stored as a one-byte boolean; they tell
whether the transition times associated with local time types were specified as standard
time or local (wall clock) time.
* tzh_ttisutcnt UT/local indicators, each stored as a one-byte boolean; they tell whether
the transition times associated with local time types were specified as UT or local
time. If a UT/local indicator is set, the corresponding standard/wall indicator must
also be set.
The standard/wall and UT/local indicators were designed for transforming a TZif file's
transition times into transitions appropriate for another time zone specified via a POSIX-
style TZ string that lacks rules. For example, when TZ="EET-2EEST" and there is no TZif
file "EET-2EEST", the idea was to adapt the transition times from a TZif file with the
well-known name "posixrules" that is present only for this purpose and is a copy of the
file "Europe/Brussels", a file with a different UT offset. POSIX does not specify this
obsolete transformational behavior, the default rules are installation-dependent, and no
implementation is known to support this feature for timestamps past 2037, so users desir-
ing (say) Greek time should instead specify TZ="Europe/Athens" for better historical cov-
erage, falling back on TZ="EET-2EEST,M3.5.0/3,M10.5.0/4" if POSIX conformance is required
and older timestamps need not be handled accurately.
The localtime(3) function normally uses the first ttinfo structure in the file 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 or for all in-
stants 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.
Version 3 format
For version-3-format timezone files, the POSIX-TZ-style string may use two minor exten-
sions 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-re-
quired 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 sav-
ing and standard time.
Interoperability considerations
Future changes to the format may append more data.
Version 1 files are considered a legacy format and should be avoided, as they do not sup-
port transition times after the year 2038. Readers that only understand Version 1 must
ignore any data that extends beyond the calculated end of the version 1 data block.
Writers should generate a version 3 file if TZ string extensions are necessary to accu-
rately model transition times. Otherwise, version 2 files should be generated.
The sequence of time changes defined by the version 1 header and data block should be a
contiguous subsequence of the time changes defined by the version 2+ header and data
block, and by the footer. This guideline helps obsolescent version 1 readers agree with
current readers about timestamps within the contiguous subsequence. It also lets writers
not supporting obsolescent readers use a tzh_timecnt of zero in the version 1 data block
to save space.
Time zone designations should consist of at least three (3) and no more than six (6) ASCII
characters from the set of alphanumerics, "-", and "+". This is for compatibility with
POSIX requirements for time zone abbreviations.
When reading a version 2 or 3 file, readers should ignore the version 1 header and data
block except for the purpose of skipping over them.
Readers should calculate the total lengths of the headers and data blocks and check that
they all fit within the actual file size, as part of a validity check for the file.
Common interoperability issues
This section documents common problems in reading or writing TZif files. Most of these
are problems in generating TZif files for use by older readers. The goals of this section
are:
* to help TZif writers output files that avoid common pitfalls in older or buggy TZif
readers,
* to help TZif readers avoid common pitfalls when reading files generated by future TZif
writers, and
* to help any future specification authors see what sort of problems arise when the TZif
format is changed.
When new versions of the TZif format have been defined, a design goal has been that a
reader can successfully use a TZif file even if the file is of a later TZif version than
what the reader was designed for. When complete compatibility was not achieved, an at-
tempt was made to limit glitches to rarely used timestamps, and to allow simple partial
workarounds in writers designed to generate new-version data useful even for older-version
readers. This section attempts to document these compatibility issues and workarounds, as
well as to document other common bugs in readers.
Interoperability problems with TZif include the following:
* Some readers examine only version 1 data. As a partial workaround, a writer can output
as much version 1 data as possible. However, a reader should ignore version 1 data, and
should use version 2+ data even if the reader's native timestamps have only 32 bits.
* Some readers designed for version 2 might mishandle timestamps after a version 3 file's
last transition, because they cannot parse extensions to POSIX in the TZ-like string.
As a partial workaround, a writer can output more transitions than necessary, so that
only far-future timestamps are mishandled by version 2 readers.
* Some readers designed for version 2 do not support permanent daylight saving time, e.g.,
a TZ string "EST5EDT,0/0,J365/25" denoting permanent Eastern Daylight Time (-04). As a
partial workaround, a writer can substitute standard time for the next time zone east,
e.g., "AST4" for permanent Atlantic Standard Time (-04).
* Some readers ignore the footer, and instead predict future timestamps from the time type
of the last transition. As a partial workaround, a writer can output more transitions
than necessary.
* Some readers do not use time type 0 for timestamps before the first transition, in that
they infer a time type using a heuristic that does not always select time type 0. As a
partial workaround, a writer can output a dummy (no-op) first transition at an early
time.
* Some readers mishandle timestamps before the first transition that has a timestamp not
less than -2**31. Readers that support only 32-bit timestamps are likely to be more
prone to this problem, for example, when they process 64-bit transitions only some of
which are representable in 32 bits. As a partial workaround, a writer can output a
dummy transition at timestamp -2**31.
* Some readers mishandle a transition if its timestamp has the minimum possible signed
64-bit value. Timestamps less than -2**59 are not recommended.
* Some readers mishandle POSIX-style TZ strings that contain "<" or ">". As a partial
workaround, a writer can avoid using "<" or ">" for time zone abbreviations containing
only alphabetic characters.
* Many readers mishandle time zone abbreviations that contain non-ASCII characters. These
characters are not recommended.
* Some readers may mishandle time zone abbreviations that contain fewer than 3 or more
than 6 characters, or that contain ASCII characters other than alphanumerics, "-", and
"+". These abbreviations are not recommended.
* Some readers mishandle TZif files that specify daylight-saving time UT offsets that are
less than the UT offsets for the corresponding standard time. These readers do not sup-
port locations like Ireland, which uses the equivalent of the POSIX TZ string
"IST-1GMT0,M10.5.0,M3.5.0/1", observing standard time (IST, +01) in summer and daylight
saving time (GMT, +00) in winter. As a partial workaround, a writer can output data for
the equivalent of the POSIX TZ string "GMT0IST,M3.5.0/1,M10.5.0", thus swapping standard
and daylight saving time. Although this workaround misidentifies which part of the year
uses daylight saving time, it records UT offsets and time zone abbreviations correctly.
Some interoperability problems are reader bugs that are listed here mostly as warnings to
developers of readers.
* Some readers do not support negative timestamps. Developers of distributed applications
should keep this in mind if they need to deal with pre-1970 data.
* Some readers mishandle timestamps before the first transition that has a nonnegative
timestamp. Readers that do not support negative timestamps are likely to be more prone
to this problem.
* Some readers mishandle time zone abbreviations like "-08" that contain "+", "-", or dig-
its.
* Some readers mishandle UT offsets that are out of the traditional range of -12 through
+12 hours, and so do not support locations like Kiritimati that are outside this range.
* Some readers mishandle UT offsets in the range [-3599, -1] seconds from UT, because they
integer-divide the offset by 3600 to get 0 and then display the hour part as "+00".
* Some readers mishandle UT offsets that are not a multiple of one hour, or of 15 minutes,
or of 1 minute.
SEE ALSO
time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8).
Olson A, Eggert P, Murchison K. The Time Zone Information Format (TZif). 2019 Feb.
Internet RFC 8536 <https://www.rfc-editor.org/info/rfc8536> doi:10.17487/RFC8536 <https://
doi.org/10.17487/RFC8536>.
COLOPHON
This page is part of release 5.10 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/.
2020-04-27 TZFILE(5)
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