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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
TZFILE(5) Linux Programmer's 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 2017,
either an ASCII NUL, or “2”, or “3”).
* 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-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 structure(s) in the file.
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_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; the second is a
signed integer specifying 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 ascending order by time. Each
transition is for one leap second, either positive or negative;
transitions always separated by at least 28 days minus 1 second.
* 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-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, 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 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,M10.5.0/3” 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 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.
Interoperability considerations
Future changes to the format may append more data.
Version 1 files are considered a legacy format and should be avoided,
as they do not support transition times after the year 2038. Readers
that only understand Version 1 must ignore any data that extends
beyond the calculated end of the version 1 data block.
Writers should generate a version 3 file if TZ string extensions are
necessary to accurately model transition times. Otherwise, version 2
files should be generated.
The sequence of time changes defined by the version 1 header and data
block should be a contiguous subsequence 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 subsequence. It also
lets writers not supporting obsolescent readers use a tzh_timecnt of
zero in the version 1 data block to save space.
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 3 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.
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 to 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 file's last transition, because they cannot parse
extensions to POSIX 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, e.g., a TZ string “EST5EDT,0/0,J365/25”
denoting permanent Eastern Daylight Time (-04). 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 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 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 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 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://www.rfc-editor.org/info/rfc8536⟩ doi:10.17487/RFC8536
⟨https://doi.org/10.17487/RFC8536⟩.
This page is part of release 5.08 of the Linux man-pages project. A
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latest version of this page, can be found at
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2020-04-27 TZFILE(5)
Pages that refer to this page: daylight(3) , timezone(3) , tzname(3) , tzset(3) , localtime(5) , tzselect(8) , zdump(8) , zic(8)
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