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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
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tzfile(5) File Formats Manual tzfile(5)
tzfile - timezone information
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.
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.
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⟩.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
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Time Zone Database tzfile(5)
Pages that refer to this page: tzset(3), localtime(5), tzselect(8), zdump(8), zic(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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