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,
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 de-
pend 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 de-
scribed 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 struc-
ture, 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 abbre-
viation bytes that follow the ttinfo entries in the file; if the
designated string is "-00", the ttinfo entry is a placeholder in-
dicating 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 val-
ues mentioned above. The byte strings can overlap if one is a
suffix of the other. The encoding of these strings is not speci-
fied.
• tzh_leapcnt pairs of four-byte values, written in network byte or-
der; 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 specify-
ing 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 previ-
ous 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; oth-
erwise, 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/lo-
cal 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.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 transforma-
tional 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="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.
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 ex-
pose 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-main-
tained timezone data (which is maintained by some other entity).
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 string in the style of the contents of a
POSIX.1-2017 TZ environment variable, for use in handling instants af-
ter 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 be-
tween the newlines) if there is no POSIX.1-2017-style representation
for such instants. If nonempty, the TZ string must agree with the lo-
cal 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 Jan-
uary 1 at 00:00 and ends December 31 at 24:00 plus the difference be-
tween 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 gen-
erated 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 possi-
ble. 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 af-
ter 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 day-
light 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 time-
stamps 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 mini-
mum 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 abbrevi-
ations are not recommended.
• Some readers mishandle TZif files that specify daylight-saving
time UT offsets that are less than the UT offsets for the corre-
sponding 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 sec-
onds that occur when the UTC offset is not a multiple of 60 sec-
onds. 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. ]8;;https://datatracker.ietf.org/doc/html/rfc8536\Internet RFC 8536]8;;\ ]8;;https://doi.org/10.17487/RFC8536\doi:10.17487/RFC8536]8;;\.
Time Zone Database tzfile(5)
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