Tribblix: manual page: tzfile.5

tzfile(5) File Formats and Configurations tzfile(5)

NAME

tzfile - timezone information

DESCRIPTION

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 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:

+o 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.

+o 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.

+o 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].

+o tzh_charcnt bytes that represent time zone designations, which
are null-terminated byte strings, each indexed by the
tt_desigidx values mentioned above. The byte strings can
overlap if one is a suffix of the other. The encoding of these
strings is not specified.

+o 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(2)) 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).

+o 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.

+o 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 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 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 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 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. 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 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
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 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.

NAME

DESCRIPTION

Interoperability considerations

Common interoperability issues

SEE ALSO