.\" 1996-06-05 by Arthur David Olson <arthur_david_olson@nih.gov>.
.\" %%%LICENSE_END
.\"
-.TH TZFILE 5 2019-03-06 "" "Linux Programmer's Manual"
+.TH TZFILE 5 2020-04-27 "" "Linux Programmer's Manual"
.SH NAME
tzfile \- timezone information
.SH DESCRIPTION
.de q
\\$3\*(lq\\$1\*(rq\\$2
..
+.ie \n(.g .ds - \f(CW-\fP
+.el ds - \-
The timezone information files used by
.BR tzset (3)
are typically found under a directory with a name like
.IR /usr/share/zoneinfo .
-These files begin with a 44-byte header containing the following fields:
+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:
.IP * 2
The magic four-byte ASCII sequence
.q "TZif"
.IP *
Fifteen bytes containing zeros reserved for future use.
.IP *
-Six four-byte integer values
-written in a standard byte order
-(the high-order byte of the value is written first).
-These values are,
-in order:
+Six four-byte integer values, in the following order:
.RS
.TP
-.I tzh_ttisgmtcnt
+.I tzh_ttisutcnt
The number of UT/local indicators stored in the file.
(UT is Universal Time.)
.TP
.IP * 2
.I tzh_timecnt
four-byte signed integer values sorted in ascending order.
-These values are written in standard byte order.
+These values are written in network byte order.
Each is used as a transition time (as returned by
.BR time (2))
at which the rules for computing local time change.
.nf
.ta .5i +\w'unsigned char\0\0'u
struct ttinfo {
- int32_t tt_gmtoff;
+ int32_t tt_utoff;
unsigned char tt_isdst;
- unsigned char tt_abbrind;
+ unsigned char tt_desigidx;
};
.in -.5i
.fi
.sp
Each structure is written as a four-byte signed integer value for
-.IR tt_gmtoff ,
-in a standard byte order, followed by a one-byte value for
+.IR tt_utoff ,
+in network byte order, followed by a one-byte boolean for
.I tt_isdst
and a one-byte value for
-.IR tt_abbrind .
+.IR tt_desigidx .
In each structure,
-.I tt_gmtoff
+.I tt_utoff
gives the number of seconds to be added to UT,
.I tt_isdst
tells whether
should be set by
.BR localtime (3)
and
-.I tt_abbrind
+.I tt_desigidx
serves as an index into the array of time zone abbreviation bytes
that follow the
.I ttinfo
structure(s) in the file.
+The
+.I tt_utoff
+value is never equal to \-2**31, to let 32-bit clients negate it without
+overflow.
+Also, in realistic applications
+.I 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].
.IP *
.I tzh_leapcnt
-pairs of four-byte values, written in standard byte order;
+pairs of four-byte values, written in network byte order;
the first value of each pair gives the nonnegative time
(as returned by
.BR time (2))
at which a leap second occurs;
-the second gives the
+the second is a signed integer specifying the
.I total
number of leap seconds to be applied during the time period
starting at the given time.
transitions always separated by at least 28 days minus 1 second.
.IP *
.I tzh_ttisstdcnt
-standard/wall indicators, each stored as a one-byte value;
+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 wall clock time,
-and are used when a timezone file is used in handling POSIX-style
-timezone environment variables.
+were specified as standard time or local (wall clock) time.
.IP *
-.I tzh_ttisgmtcnt
-UT/local indicators, each stored as a one-byte value;
+.I 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,
-and are used when a timezone file is used in handling POSIX-style
-timezone environment variables.
+were specified as UT or local time.
+If a UT/local indicator is set, the corresponding standard/wall indicator
+must also be set.
+.PP
+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.
.PP
The
.BR localtime (3)
function
-uses the first standard-time
+normally uses the first
.I ttinfo
structure in the file
-(or simply the first
-.I ttinfo
-structure in the absence of a standard-time structure)
if either
.I tzh_timecnt
is zero or the time argument is less than the first transition time recorded
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.
-.PP
+.SS Interoperability considerations
Future changes to the format may append more data.
+.PP
+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.
+.PP
+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.
+.PP
+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
+.I tzh_timecnt
+of zero
+in the version 1 data block to save space.
+.PP
+Time zone designations should consist of at least three (3)
+and no more than six (6) ASCII characters from the set of
+alphanumerics,
+.q "\*-",
+and
+.q "+".
+This is for compatibility with POSIX requirements for
+time zone abbreviations.
+.PP
+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.
+.PP
+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.
+.SS 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:
+.IP * 2
+to help TZif writers output files that avoid common
+pitfalls in older or buggy TZif readers,
+.IP *
+to help TZif readers avoid common pitfalls when reading
+files generated by future TZif writers, and
+.IP *
+to help any future specification authors see what sort of
+problems arise when the TZif format is changed.
+.PP
+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.
+.PP
+Interoperability problems with TZif include the following:
+.IP * 2
+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.
+.IP *
+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.
+.IP *
+Some readers designed for version 2 do not support
+permanent daylight saving time, e.g., a TZ string
+.q "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.,
+.q "AST4"
+for permanent Atlantic Standard Time (\-04).
+.IP *
+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.
+.IP *
+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.
+.IP *
+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.
+.IP *
+Some readers mishandle a transition if its timestamp has
+the minimum possible signed 64-bit value.
+Timestamps less than \-2**59 are not recommended.
+.IP *
+Some readers mishandle POSIX-style TZ strings that
+contain
+.q "<"
+or
+.q ">".
+As a partial workaround, a writer can avoid using
+.q "<"
+or
+.q ">"
+for time zone abbreviations containing only alphabetic
+characters.
+.IP *
+Many readers mishandle time zone abbreviations that contain
+non-ASCII characters.
+These characters are not recommended.
+.IP *
+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,
+.q "\*-",
+and
+.q "+".
+These abbreviations are not recommended.
+.IP *
+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
+.q "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
+.q "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.
+.PP
+Some interoperability problems are reader bugs that
+are listed here mostly as warnings to developers of readers.
+.IP * 2
+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.
+.IP *
+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.
+.IP *
+Some readers mishandle time zone abbreviations like
+.q "\*-08"
+that contain
+.q "+",
+.q "\*-",
+or digits.
+.IP *
+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.
+.IP *
+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
+.q "+00".
+.IP *
+Some readers mishandle UT offsets that are not a multiple
+of one hour, or of 15 minutes, or of 1 minute.
.SH SEE ALSO
.BR time (2),
.BR localtime (3),
.BR tzset (3),
.BR tzselect (8),
.BR zdump (8),
-.BR zic (8)
+.BR zic (8).
+.PP
+Olson A, Eggert P, Murchison K. The Time Zone Information Format (TZif).
+2019 Feb.
+.UR https://\:www.rfc-editor.org/\:info/\:rfc8536
+Internet RFC 8536
+.UE
+.UR https://\:doi.org/\:10.17487/\:RFC8536
+doi:10.17487/RFC8536
+.UE .
.\" This file is in the public domain, so clarified as of
.\" 1996-06-05 by Arthur David Olson.
projects / thirdparty / man-pages.git / commitdiff
