1TZFILE(5) Linux Programmer's Manual TZFILE(5)
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6 tzfile - timezone information
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9 The timezone information files used by tzset(3) are typically found
10 under a directory with a name like /usr/share/zoneinfo. These files
11 begin with a 44-byte header containing the following fields:
12 * The magic four-byte ASCII sequence “TZif” identifies the file as a
14 timezone information file.
15 * A byte identifying the version of the file's format (as of 2017,
17 either an ASCII NUL, or “2”, or “3”).
18 * Fifteen bytes containing zeros reserved for future use.
20 * Six four-byte integer values written in a standard byte order (the
22 high-order byte of the value is written first). These values are, in
23 order:
24 tzh_ttisgmtcnt
26 The number of UT/local indicators stored in the file.
27 tzh_ttisstdcnt
29 The number of standard/wall indicators stored in the file.
30 tzh_leapcnt
32 The number of leap seconds for which data entries are stored
33 in the file.
34 tzh_timecnt
36 The number of transition times for which data entries are
37 stored in the file.
38 tzh_typecnt
40 The number of local time types for which data entries are
41 stored in the file (must not be zero).
42 tzh_charcnt
44 The number of bytes of timezone abbreviation strings stored in
45 the file.
46 The above header is followed by the following fields, whose lengths
48 vary depend on the contents of the header:
49 * tzh_timecnt four-byte signed integer values sorted in ascending
51 order. These values are written in standard byte order. Each is
52 used as a transition time (as returned by time(2)) at which the rules
53 for computing local time change.
54 * tzh_timecnt one-byte unsigned integer values; each one tells which of
56 the different types of local time types described in the file is
57 associated with the time period starting with the same-indexed tran‐
58 sition time. These values serve as indices into the next field.
59 * tzh_typecnt ttinfo entries, each defined as follows:
61 struct ttinfo {
63 int32_t tt_gmtoff;
64 unsigned char tt_isdst;
65 unsigned char tt_abbrind;
66 };
67 Each structure is written as a four-byte signed integer value for
69 tt_gmtoff, in a standard byte order, followed by a one-byte value for
70 tt_isdst and a one-byte value for tt_abbrind. In each structure,
71 tt_gmtoff gives the number of seconds to be added to UT, tt_isdst tells
72 whether tm_isdst should be set by localtime(3) and tt_abbrind serves as
73 an index into the array of timezone abbreviation bytes that follow the
74 ttinfo structure(s) in the file.
75 * tzh_leapcnt pairs of four-byte values, written in standard byte
77 order; the first value of each pair gives the nonnegative time
78 (as returned by time(2)) at which a leap second occurs; the sec‐
79 ond gives the total number of leap seconds to be applied during
80 the time period starting at the given time. The pairs of values
81 are sorted in ascending order by time. Each transition is for
82 one leap second, either positive or negative; transitions always
83 separated by at least 28 days minus 1 second.
84 * tzh_ttisstdcnt standard/wall indicators, each stored as a one-
86 byte value; they tell whether the transition times associated
87 with local time types were specified as standard time or wall
88 clock time, and are used when a timezone file is used in han‐
89 dling POSIX-style timezone environment variables.
90 * tzh_ttisgmtcnt UT/local indicators, each stored as a one-byte
92 value; they tell whether the transition times associated with
93 local time types were specified as UT or local time, and are
94 used when a timezone file is used in handling POSIX-style time‐
95 zone environment variables.
96 The localtime(3) function uses the first standard-time ttinfo structure
98 in the file (or simply the first ttinfo structure in the absence of a
99 standard-time structure) if either tzh_timecnt is zero or the time
100 argument is less than the first transition time recorded in the file.
101 Version 2 format
103 For version-2-format timezone files, the above header and data are fol‐
104 lowed by a second header and data, identical in format except that
105 eight bytes are used for each transition time or leap second time.
106 (Leap second counts remain four bytes.) After the second header and
107 data comes a newline-enclosed, POSIX-TZ-environment-variable-style
108 string for use in handling instants after the last transition time
109 stored in the file (with nothing between the newlines if there is no
110 POSIX representation for such instants). The POSIX-style string must
111 agree with the local time type after both data's last transition times;
112 for example, given the string “WET0WEST,M3.5.0,M10.5.0/3” then if a
113 last transition time is in July, the transition's local time type must
114 specify a daylight-saving time abbreviated “WEST” that is one hour east
115 of UT.
116 Version 3 format
118 For version-3-format timezone files, the POSIX-TZ-style string may use
119 two minor extensions to the POSIX TZ format, as described in
120 newtzset(3). First, the hours part of its transition times may be
121 signed and range from -167 through 167 instead of the POSIX-required
122 unsigned values from 0 through 24. Second, DST is in effect all year
123 if it starts January 1 at 00:00 and ends December 31 at 24:00 plus the
124 difference between daylight saving and standard time.
125 Future changes to the format may append more data.
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129 time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8)
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132 This page is part of release 4.16 of the Linux man-pages project. A
133 description of the project, information about reporting bugs, and the
134 latest version of this page, can be found at
135 https://www.kernel.org/doc/man-pages/.
136 2017-08-04 TZFILE(5)