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+.TH TZFILE 5
+.SH NAME
+tzfile \- time zone information
+.SH DESCRIPTION
+.ie '\(lq'' .ds lq \&"\"
+.el .ds lq \(lq\"
+.ie '\(rq'' .ds rq \&"\"
+.el .ds rq \(rq\"
+.de q
+\\$3\*(lq\\$1\*(rq\\$2
+..
+The time zone 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:
+.IP * 2
+The magic four-byte ASCII sequence
+.q "TZif"
+identifies the file as a time zone information file.
+.IP *
+A byte identifying the version of the file's format
+(as of 2017, either an ASCII NUL, or
+.q "2",
+or
+.q "3" ).
+.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:
+.RS
+.TP
+.I tzh_ttisgmtcnt
+The number of UT/local indicators stored in the file.
+.TP
+.I tzh_ttisstdcnt
+The number of standard/wall indicators stored in the file.
+.TP
+.I tzh_leapcnt
+The number of leap seconds for which data entries are stored in the file.
+.TP
+.I tzh_timecnt
+The number of transition times for which data entries are stored
+in the file.
+.TP
+.I tzh_typecnt
+The number of local time types for which data entries are stored
+in the file (must not be zero).
+.TP
+.I tzh_charcnt
+The number of bytes of time zone abbreviation strings
+stored in the file.
+.RE
+.PP
+The above header is followed by the following fields, whose lengths
+depend on the contents of the header:
+.IP * 2
+.I tzh_timecnt
+four-byte signed integer values sorted in ascending order.
+These values are written in standard byte order.
+Each is used as a transition time (as returned by
+.BR time (2))
+at which the rules for computing local time change.
+.IP *
+.I tzh_timecnt
+one-byte unsigned integer values;
+each one 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.
+These values serve as indices into the next field.
+.IP *
+.I tzh_typecnt
+.I ttinfo
+entries, each defined as follows:
+.in +.5i
+.sp
+.nf
+.ta .5i +\w'unsigned char\0\0'u
+struct ttinfo {
+	int32_t	tt_gmtoff;
+	unsigned char	tt_isdst;
+	unsigned char	tt_abbrind;
+};
+.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
+.I tt_isdst
+and a one-byte value for
+.IR tt_abbrind .
+In each structure,
+.I tt_gmtoff
+gives the number of seconds to be added to UT,
+.I tt_isdst
+tells whether
+.I tm_isdst
+should be set by
+.BR localtime (3)
+and
+.I tt_abbrind
+serves as an index into the array of time zone abbreviation bytes
+that follow the
+.I ttinfo
+structure(s) in the file.
+.IP *
+.I tzh_leapcnt
+pairs of four-byte values, written in standard 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
+.I 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.
+.IP *
+.I tzh_ttisstdcnt
+standard/wall indicators, each stored as a one-byte value;
+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 time zone file is used in handling POSIX-style
+time zone environment variables.
+.IP *
+.I tzh_ttisgmtcnt
+UT/local indicators, each stored as a one-byte value;
+they tell whether the transition times associated with local time types
+were specified as UT or local time,
+and are used when a time zone file is used in handling POSIX-style
+time zone environment variables.
+.PP
+The
+.BR localtime (3)
+function
+uses the first standard-time
+.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
+in the file.
+.SS Version 2 format
+For version-2-format time zone 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
+(with nothing between the newlines if there is no POSIX representation for
+such instants).
+The POSIX-style string must agree with the local time type after
+both data's last transition times; for example, given the string
+.q "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
+.q "WEST"
+that is one hour east of UT.
+.SS Version 3 format
+For version-3-format time zone files, the POSIX-TZ-style string may
+use two minor extensions to the POSIX TZ format, as described in
+.BR 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.
+.PP
+Future changes to the format may append more data.
+.SH SEE ALSO
+.BR time (2),
+.BR localtime (3),
+.BR tzset (3),
+.BR tzselect (8),
+.BR zdump (8),
+.BR zic (8)
+.\" This file is in the public domain, so clarified as of
+.\" 1996-06-05 by Arthur David Olson.