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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 se‐
quence 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 bool‐
ean 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 dif‐
ferent 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 including 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_desigidx.
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 structure(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 ap‐
plied 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 nega‐
tive; 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 tran‐
sition 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 "Eu‐
rope/Brussels", a file with a different UT offset. POSIX does not specify this obsolete
transformational behavior, the default rules are installation-dependent, and no implementa‐
tion is known to support this feature for timestamps past 2037, so users desiring (say) Greek
time should instead specify TZ="Europe/Athens" for better historical coverage, 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 ac‐
tually 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 han‐
dling instants after the last transition time stored in the file or for all instants if the
file has no transitions. The POSIX-style TZ string is empty (i.e., nothing between the new‐
lines) 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 transi‐
tion, 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 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.

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 support
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 accurately
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 con‐
tiguous 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 ob‐
solescent 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 read‐
ers,

* 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 for‐
mat 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 attempt was made
to limit glitches to rarely used timestamps, and to allow simple partial workarounds in writ‐
ers designed to generate new-version data useful even for older-version readers. This sec‐
tion 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 par‐
tial 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 rep‐
resentable 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 work‐
around, a writer can avoid using “<” or “>” for time zone abbreviations containing only al‐
phabetic 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 support
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 de‐
velopers 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 time‐
stamp. 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 digits.

* 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
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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)