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1 .\" $NetBSD: tzfile.5,v 1.23 2015/10/09 17:21:45 christos Exp $
2 .\"
3 .\" This file is in the public domain, so clarified as of
4 .\" 1996-06-05 by Arthur David Olson (arthur_david_olson@nih.gov).
5 .Dd October 6, 2014
6 .Dt TZFILE 5
7 .Os
8 .Sh NAME
9 .Nm tzfile
10 .Nd time zone information
11 .Sh DESCRIPTION
12 The time zone information files used by
13 .Xr tzset 3
14 begin with the magic characters
15 .Dq TZif
16 to identify them as time zone information files,
17 followed by a character identifying the version of the file's format
18 (as of 2013, either an ASCII NUL or a '2', or '3')
19 followed by fifteen bytes containing zeroes reserved for future use,
20 followed by six four-byte values of type
21 .Fa long ,
22 followed by six four-byte integer values written in a standard
23 byte order (the high-order byte of the value is written first).
24 These values are, in order:
25 .Bl -tag -width XXXXXX -compact
26 .It Va tzh_ttisgmtcnt
27 The number of UT/local indicators stored in the file.
28 .It Va tzh_ttisstdcnt
29 The number of standard/wall indicators stored in the file.
30 .It Va tzh_leapcnt
31 The number of leap seconds for which data entries are stored in the file.
32 .It Va tzh_timecnt
33 The number of transition times for which data entries are stored
34 in the file.
35 .It Va tzh_typecnt
36 The number of local time types for which data entries are stored
37 in the file (must not be zero).
38 .It Va tzh_charcnt
39 The number of characters of time zone abbreviation strings
40 stored in the file.
41 .El
42 .Pp
43 The above header is followed by
44 .Va tzh_timecnt
45 four-byte signed integer values sorted in ascending order.
46 These values are written in
47 These values are written in standard byte order.
48 Each is used as a transition time (as returned by
49 .Xr time 3 )
50 at which the rules for computing local time change.
51 Next come
52 .Va tzh_timecnt
53 one-byte unsigned integer values;
54 each one tells which of the different types of local time types
55 described in the file is associated with the time period
56 starting with the same-indexed transition time.
57 These values serve as indices into an array of
58 .Fa ttinfo
59 structures (with
60 .Va tzh_typecnt
61 entries) that appears next in the file;
62 these structures are defined as follows:
63 .Bd -literal
64 struct ttinfo {
65 int32_t tt_gmtoff;
66 unsigned char tt_isdst;
67 unsigned char tt_abbrind;
68 };
69 .Ed
70 Each structure is written as a four-byte signed integer value for
71 .Va tt_gmtoff
72 in a standard byte order, followed by a one-byte value for
73 .Va tt_isdst
74 and a one-byte value for
75 .Va tt_abbrind .
76 In each structure,
77 .Va tt_gmtoff
78 gives the number of seconds to be added to UT,
79 .Va tt_isdst
80 tells whether
81 .Va tm_isdst
82 should be set by
83 .Xr localtime 3
84 and
85 .Va tt_abbrind
86 serves as an index into the array of time zone abbreviation characters
87 that follow the
88 .Va ttinfo
89 structure(s) in the file.
90 .Pp
91 Then there are
92 .Va tzh_leapcnt
93 pairs of four-byte values, written in standard byte order;
94 the first value of each pair gives the time
95 (as returned by
96 .Xr time 3 )
97 at which a leap second occurs;
98 the second gives the
99 .Em total
100 number of leap seconds to be applied during the time period
101 starting at the given time.
102 The pairs of values are sorted in ascending order by time.
103 .Pp
104 Then there are
105 .Va tzh_ttisstdcnt
106 standard/wall indicators, each stored as a one-byte value;
107 they tell whether the transition times associated with local time types
108 were specified as standard time or wall clock time,
109 and are used when a time zone file is used in handling POSIX-style
110 time zone environment variables.
111 .Pp
112 Finally there are
113 .Va tzh_ttisgmtcnt
114 UT/local indicators, each stored as a one-byte value;
115 they tell whether the transition times associated with local time types
116 were specified as UT or local time,
117 and are used when a time zone file is used in handling POSIX-style
118 time zone environment variables.
119 .Pp
120 .Xr localtime 3
121 uses the first standard-time
122 .Fa ttinfo
123 structure in the file
124 (or simply the first
125 .Fa ttinfo
126 structure in the absence of a standard-time structure)
127 if either
128 .Va tzh_timecnt
129 is zero or the time argument is less than the first transition time recorded
130 in the file.
131 .Pp
132 For version-2-format time zone files,
133 the above header and data are followed by a second header and data,
134 identical in format except that
135 eight bytes are used for each transition time or leap second time.
136 After the second header and data comes a newline-enclosed,
137 POSIX-TZ-environment-variable-style string for use in handling instants
138 after the last transition time stored in the file
139 (with nothing between the newlines if there is no POSIX representation for
140 such instants).
141 .Pp
142 For version-3-format time zone files, the POSIX-TZ-style string may
143 use two minor extensions to the POSIX TZ format, as described in
144 .Xr tzset 3 .
145 First, the hours part of its transition times may be signed and range from
146 \-167 through 167 instead of the POSIX-required unsigned values
147 from 0 through 24.
148 Second, DST is in effect all year if it starts
149 January 1 at 00:00 and ends December 31 at 24:00 plus the difference
150 between daylight saving and standard time.
151 .Pp
152 Future changes to the format may append more data.
153 .Sh SEE ALSO
154 .Xr ctime 3 ,
155 .Xr localtime 3 ,
156 .Xr time 3 ,
157 .Xr tzset 3 ,
158 .Xr zdump 8
159 .Xr zic 8
160 .\" @(#)tzfile.5 8.3
161 .\" This file is in the public domain, so clarified as of
162 .\" 1996-06-05 by Arthur David Olson.