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.
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).
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.
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.
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.
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
doi:10.17487/RFC8536