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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 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 2021, either 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 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 including the next transition time. (The last time type is present only for
consistency checking with the POSIX.1-2017-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 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.1-2017-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
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 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 string in the style of the contents of a
POSIX.1-2017 TZ environment variable, 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 TZ string is empty (i.e., nothing between the newlines) if there is no
POSIX.1-2017-style representation for such instants. If nonempty, the 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 TZ string may use two minor extensions to the
POSIX.1-2017 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 expiration, 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 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.1-2017
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
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 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 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 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.
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⟩.