TZFILE(5) FreeBSD File Formats Manual TZFILE(5)
NAME
tzfile - timezone information
DESCRIPTION
The timezone information files used by tzset(3) are found under
/usr/share/zoneinfo. These files use the format described in Internet
RFC 9636. 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:
o The magic four-byte ASCII sequence "TZif" identifies the file as a
timezone information file.
o A byte identifying the version of the file's format (as of 2021,
either an ASCII NUL, "2", "3", or "4").
o Fifteen bytes containing zeros reserved for future use.
o 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 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 proleptic 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, and each corresponding to a time zone
abbreviation. 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 non-negative 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 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 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 proleptic 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 was present only for this purpose and was a
copy of the file "Europe/Brussels", a file with a different UT offset.
POSIX does not specify the details of 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 conformance to POSIX.1-2017 or
earlier 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 proleptic TZ,
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 proleptic
representation for such instants. If non-empty, 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. The TZ string can
contain time zone abbreviations and UT offsets that do not appear
elsewhere in the TZif file. 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, a TZ string (see newtzset(3)) may
use the following POSIX.1-2024 extensions to POSIX.1-2017: First, as in
TZ="<-02>2<-01>,M3.5.0/-1,M10.5.0/0", the hours part of its transition
times may be signed and range from -167 through 167 instead of being
limited to unsigned values from 0 through 24. Second, as in
TZ="XXX3EDT4,0/0,J365/23", 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 abbreviations 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.
A numeric time zone abbreviation should match the UT offset. For
example, "+0530" should be used only if the UT offset is 5.5 hours ahead
of UT, and "-00" should be used only if the UT offset is zero.
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:
o TZif writers output files that avoid common pitfalls in older or
buggy TZif readers,
o TZif readers avoid common pitfalls when reading files generated by
future TZif writers, and
o 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 newer-version data useful even for older-
version readers. This section attempts to document these compatibility
issues and workarounds as well as documenting other common bugs in
readers.
Interoperability problems with TZif include the following:
o 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.
o Some readers designed for version 2 might mishandle timestamps after
a version 3 or higher file's last transition, because they cannot
parse the POSIX.1-2024 extensions to POSIX.1-2017 in the proleptic TZ
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.
o Some readers might mishandle timestamps after a file's last
transition, because they require that all abbreviations or UT offsets
in the proleptic TZ string must also occur somewhere in the file's
tables of time zone designations and local time type records. As a
workaround, a writer can output more transitions than necessary, so
that the other tables contain duplicates of the proleptic TZ string's
abbreviations and offsets.
o 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).
o Some readers designed for version 2 or 3 and that require strict
conformance to RFC 9636 reject version 4 files whose leap second
tables are truncated at the start or end in expiration times.
o 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.
o Some stripped-down readers ignore everything but the footer, and use
its proleptic TZ string to calculate all timestamps. Although this
approach often works for current and future timestamps, it obviously
has problems with past timestamps, and even for current timestamps it
can fail for settings like TZ="Africa/Casablanca". This corresponds
to a TZif file containing explicit transitions through the year 2087,
followed by a footer containing the TZ string "<+01>-1", which should
be used only for timestamps after the last explicit transition.
o 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.
o Some readers mishandle timestamps before the first transition that
has a timestamp that is 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.
o Some readers mishandle a transition if its timestamp has the minimum
possible signed 64-bit value. Timestamps less than -2**59 are not
recommended.
o Some readers mishandle proleptic TZ strings that contain "<" or ">".
As a partial workaround, a writer can avoid using "<" or ">" for time
zone abbreviations containing only alphabetic characters.
o Many readers mishandle time zone abbreviations that contain non-ASCII
characters. These characters are not recommended.
o 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.
o 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.
o Some readers generate ambiguous timestamps for positive leap seconds
that occur when the UTC offset is not a multiple of 60 seconds. For
example, with UTC offset +01:23:45 and 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.
o 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.
o Some readers mishandle timestamps before the first transition that
has a non-negative timestamp. Readers that do not support negative
timestamps are likely to be more prone to this problem.
o Some readers mishandle time zone abbreviations like "-08" that
contain "+", "-", or digits.
o 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.
o 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".
o Some readers mishandle UT offsets that are not a multiple of one
hour, or of 15 minutes, or of 1 minute.
SEE ALSO
time(3), localtime(3), tzset(3), tzsetup(8), zic(8), zdump(8)
A. Olson, P. Eggert, and K. Murchison, The Time Zone Information Format
(TZif), RFC 9636, https://datatracker.ietf.org/doc/html/rfc9636,
https://doi.org/10.17487/RFC9636, October 2024.
FreeBSD 15.1-STABLE-HBSD March 8, 2026 TZFILE(5)