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.
