# tzfile

## 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:
    
             •  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
                2021, either an ASCII NUL, “2”, “3”, or “4”).
    
             •  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 proleptic 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**
                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].
    
             •  **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.
    
             •  **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).
    
             •  **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 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:
    
             •  TZif writers output files that avoid common pitfalls in older
                or buggy TZif readers,
    
             •  TZif readers avoid common pitfalls when reading files
                generated by future TZif writers, and
    
             •  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:
    
             •  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 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.
    
             •  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).
    
             •  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.
    
             •  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 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.
    
             •  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 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.
    
             •  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 proleptic 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 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.
    
             •  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.
    
             •  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 non-negative 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.
    


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