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tzfile(5) File Formats Manual tzfile(5)
NAME
tzfile - timezone information
DESCRIPTION
The timezone information files used by tzset(3) are typically found un-
der 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 repre-
sented 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 2021, ei-
ther an ASCII NUL, "2", "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 time zone abbreviation strings stored
in the file.
The above header is followed by the following fields, whose lengths de-
pend on the contents of the header:
* tzh_timecnt four-byte signed integer values sorted in ascending or-
der. 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 including the
next transition time. (The last time type is present only for con-
sistency 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 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 nonnegative time (as returned
by time(2)) at which a leap second occurs or at which the leap second
table expires; the second is a signed integer specifying the correc-
tion, 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 de-
notes 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 non-
negative 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 correc-
tion 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 transform-
ing a TZif file's transition times into transitions appropriate for an-
other 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 behav-
ior, 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="Eu-
rope/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.
Version 2 format
For version-2-format timezone files, the above header and data are fol-
lowed 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 spec-
ify 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 associ-
ated with the time period from the indefinite past up to but not in-
cluding 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.
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 ex-
piration, and the added leap seconds will change how post-expiration
timestamps are treated.
Interoperability considerations
Future changes to the format may append more data.
Version 1 files are considered a legacy format and should not be gener-
ated, as they do not support transition times after the year 2038.
Readers that understand only Version 1 must ignore any data that ex-
tends 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 ver-
sion 4 file only if its leap second table either expires or is trun-
cated at the start. Likewise, a writer not generating a version 4 file
should generate a version 3 file only if TZ string extensions are nec-
essary 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 read-
ers 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 ver-
sion 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 sec-
ond to the local minute containing the second just before the leap sec-
ond. If this occurs when the UTC offset is not a multiple of 60 sec-
onds, 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 gener-
ated 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 work-
arounds 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 com-
mon 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 work-
around, 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 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 ta-
bles 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 out-
put 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 day-
light 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 in-
stead 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 practi-
cal 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 dis-
tributed 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 con-
tain "+", "-", 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://datatracker.ietf.org/
doc/html/rfc8536> doi:10.17487/RFC8536 <https://doi.org/10.17487/
RFC8536>.
Time Zone Database tzfile(5)
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