Updated: 2022/Sep/29
Please read Privacy Policy. It's for your privacy.
TZFILE(5) File Formats Manual TZFILE(5)
NAME
tzfile - time zone 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 magic four-byte ASCII sequence begin with the magic characters
"TZif". identifies the file as a timezone information file.
⊕ A byte identifying the version of the file's format (as of 2021,
either an ASCII NUL, or "3", or "4 )."
⊕ 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 timezone 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 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 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_uttoff;
unsigned char tt_isdst;
unsigned char tt_desigind;
};
Each structure is written as a four-byte signed integer value for
tt_gmtoff in a network byte order, followed by a one-byte value for
tt_isdst and a one-byte value for tt_desigidx. 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_desigidx
serves as an index into the array of timezone abbreviation bytes that
follow the ttinfo structure(s) in the file. entries in the file; if
the designated string is "\*-00", the ttinfo entry is a placeholder
indicating that local time is unspecified. 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_charcnt bytes that represent time zone designations, which are
null-terminated byte strings, each indexed by the
tt_desigidx values mentioned above. The byte strings can overlap if
one is a suffix of the other. The encoding of these strings is not
specified.
tzh_leapcnt pairs of four-byte values, written in network byte order;
the first value of each pair gives the time (as returned by time(3))
at which a leap second occurs or at which the leap second table
expires; the second is a signed integer specifying the correction,
which is 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 strictly ascending order by time. Each pair denotes one
leap second, either positive or negative, except that if the last
pair has the same correction as the previous one, the last pair
denotes the leap second table's expiration time. Each leap second is
at the end of a UTC calendar month. The first leap second has a
nonnegative occurrence time, and is a positive leap second if and
only if its correction is positive; the correction for each leap
second after the first differs from the previous leap second by
either 1 for a positive leap second, or -1 for a negative leap
second. If the leap second table is empty, the leap-second
correction is zero for all timestamps; otherwise, for timestamps
before the first occurrence time, the leap-second correction is zero
if the first pair's correction is 1 or -1, and is unspecified
otherwise (which can happen only in files truncated at the start).
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="EET2EEST" and there
is no TZif file "EET2EEST", 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 desiring (say) Greek
time should instead specify TZ="Europe/Athens" for better historical
coverage, falling back on TZ="EET2EEST,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.
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 instants if the file has no transitions. The POSIX-style TZ
string is empty (i.e., nothing between the newlines) if there is no
POSIX-style 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,M3.5.0/1,M10.5.0" 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 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.
Version 4 format
For version-4-format TZif files, the first leap second record can have a
correction that is neither +1 nor -1, to represent truncation of the TZif
file at the start. Also, if two or more leap second transitions are
present and the last entry's correction equals the previous one, the last
entry denotes the expiration of the leap second table instead of a leap
second; timestamps after this expiration are unreliable in that future
releases will likely add leap second entries after the expiration, and
the added leap seconds will change how post-expiration timestamps are
treated.
Interoperability considerations
Version 1 files are considered a legacy format and should not be
generated, as they do not support transition times after the year 2038.
Readers that understand only Version 1 must ignore any data that extends
beyond the calculated end of the version 1 data block. Other than
version 1, writers should generate the lowest version number needed by a
file's data. For example, a writer should generate a version 4 file only
if its leap second table either expires or is truncated at the start.
Likewise, a writer not generating a version 4 file should generate a
version 3 file only if TZ string extensions are necessary to accurately
model transition times.
The sequence of time changes defined by the version 1 header and data
block should be a contiguous sub-sequence 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 sub-sequence. It also lets writers not
supporting obsolescent readers use a tzh_timecnt of zero in the version 1
data block to save space.
When a TZif file contains a leap second table expiration time, TZif
readers should either refuse to process post-expiration timestamps, or
process them as if the expiration time did not exist (possibly with an
error indication).
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 higher 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.
When a positive leap second occurs, readers should append an extra second
to the local minute containing the second just before the leap second.
If this occurs when the UTC offset is not a multiple of 60 seconds, the
leap second occurs earlier than the last second of the local minute and
the minute's remaining local seconds are numbered through 60 instead of
the usual 59; the UTC offset is unaffected.
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 attempt was made to limit
glitches to rarely used timestamps and 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 or higher 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 permanent daylight saving time with
transitions after 24:00 - e.g., a TZ string "EST5EDT,0/0,J365/25"
denoting permanent Eastern Daylight Time (-04). As a workaround, a
writer can substitute standard time for two time zones east, e.g.,
"XXX3EDT4,0/0,J365/23" for a time zone with a never-used standard
time (XXX, -03) and negative daylight saving time (EDT, -04) all
year. Alternatively, 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 designed for version 2 or 3, and that require strict
conformance to RFC 8536, reject version 4 files whose leap second
tables are truncated at the start or that end in expiration times.
⊕ 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 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 readers generate ambiguous timestamps for positive leap seconds
that occur when the UTC offset is not a multiple of 60 seconds. For
example, in a timezone with UTC offset +01:23:45 and with a positive
leap second 78796801 (1972-06-30 23:59:60 UTC), some readers will map
both 78796800 and 78796801 to 01:23:45 local time the next day
instead of mapping the latter to 01:23:46, and they will map 78796815
to 01:23:59 instead of to 01:23:60. This has not yet been a
practical problem, since no civil authority has observed such UTC
offsets since leap seconds were introduced in 1972.
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 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. Future changes to the format
may append more data.
SEE ALSO
ctime(3), localtime(3), time(3), tzset(3), zdump(8), zic(8).
Olson A, Eggert P, Murchison K., The Time Zone Information Format
(TZif)., RFC 8536, https://datatracker.ietf.org/doc/html/rfc8536,
https://doi.org/10.17487/RFC8536, Feb 2019..
NetBSD 10.99 September 16, 2023 NetBSD 10.99