1TZSET(3) Linux Programmer's Manual TZSET(3)
2
6 tzset, tzname, timezone, daylight - initialize time conversion informa‐
7 tion
8
10 #include <time.h>
11 void tzset (void);
13 extern char *tzname[2];
15 extern long timezone;
16 extern int daylight;
17 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
19 tzset(): _POSIX_C_SOURCE
21 tzname: _POSIX_C_SOURCE
22 timezone, daylight: _XOPEN_SOURCE
23 || /* Glibc since 2.19: */ _DEFAULT_SOURCE
24 || /* Glibc versions <= 2.19: */ _SVID_SOURCE
25
27 The tzset() function initializes the tzname variable from the TZ envi‐
28 ronment variable. This function is automatically called by the other
29 time conversion functions that depend on the timezone. In a System-V-
30 like environment, it will also set the variables timezone (seconds West
31 of UTC) and daylight (to 0 if this timezone does not have any daylight
32 saving time rules, or to nonzero if there is a time, past, present or
33 future when daylight saving time applies).
34 If the TZ variable does not appear in the environment, the system time‐
36 zone is used. The system timezone is configured by copying, or link‐
37 ing, a file in the tzfile(5) format to /etc/localtime. A timezone
38 database of these files may be located in the system timezone directory
39 (see the FILES section below).
40 If the TZ variable does appear in the environment, but its value is
42 empty, or its value cannot be interpreted using any of the formats
43 specified below, then Coordinated Universal Time (UTC) is used.
44 The value of TZ can be one of two formats. The first format is a
46 string of characters that directly represent the timezone to be used:
47 std offset[dst[offset][,start[/time],end[/time]]]
49 There are no spaces in the specification. The std string specifies an
51 abbreviation for the timezone and must be three or more alphabetic
52 characters. When enclosed between the less-than (<) and greater-than
53 (>) signs, the characters set is expanded to include the plus (+) sign,
54 the minus (-) sign, and digits. The offset string immediately follows
55 std and specifies the time value to be added to the local time to get
56 Coordinated Universal Time (UTC). The offset is positive if the local
57 timezone is west of the Prime Meridian and negative if it is east. The
58 hour must be between 0 and 24, and the minutes and seconds 00 and 59:
59 [+|-]hh[:mm[:ss]]
61 The dst string and offset specify the name and offset for the corre‐
63 sponding daylight saving timezone. If the offset is omitted, it de‐
64 faults to one hour ahead of standard time.
65 The start field specifies when daylight saving time goes into effect
67 and the end field specifies when the change is made back to standard
68 time. These fields may have the following formats:
69 Jn This specifies the Julian day with n between 1 and 365. Leap
71 days are not counted. In this format, February 29 can't be rep‐
72 resented; February 28 is day 59, and March 1 is always day 60.
73 n This specifies the zero-based Julian day with n between 0 and
75 365. February 29 is counted in leap years.
76 Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w <= 5) of
78 month m (1 <= m <= 12). Week 1 is the first week in which day d
79 occurs and week 5 is the last week in which day d occurs. Day 0
80 is a Sunday.
81 The time fields specify when, in the local time currently in effect,
83 the change to the other time occurs. If omitted, the default is
84 02:00:00.
85 Here is an example for New Zealand, where the standard time (NZST) is
87 12 hours ahead of UTC, and daylight saving time (NZDT), 13 hours ahead
88 of UTC, runs from the first Sunday in October to the third Sunday in
89 March, and the changeovers happen at the default time of 02:00:00:
90 TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
92 The second format specifies that the timezone information should be
94 read from a file:
95 :[filespec]
97 If the file specification filespec is omitted, or its value cannot be
99 interpreted, then Coordinated Universal Time (UTC) is used. If file‐
100 spec is given, it specifies another tzfile(5)-format file to read the
101 timezone information from. If filespec does not begin with a '/', the
102 file specification is relative to the system timezone directory. If
103 the colon is omitted each of the above TZ formats will be tried.
104 Here's an example, once more for New Zealand:
106 TZ=":Pacific/Auckland"
108
110 TZ If this variable is set its value takes precedence over the sys‐
111 tem configured timezone.
112 TZDIR If this variable is set its value takes precedence over the sys‐
114 tem configured timezone database directory path.
115
117 /etc/localtime
118 The system timezone file.
119 /usr/share/zoneinfo/
121 The system timezone database directory.
122 /usr/share/zoneinfo/posixrules
124 When a TZ string includes a dst timezone without anything fol‐
125 lowing it, then this file is used for the start/end rules. It
126 is in the tzfile(5) format. By default, the zoneinfo Makefile
127 hard links it to the America/New_York tzfile.
128 Above are the current standard file locations, but they are config‐
130 urable when glibc is compiled.
131
133 For an explanation of the terms used in this section, see at‐
134 tributes(7).
135 ┌──────────┬───────────────┬────────────────────┐
137 │Interface │ Attribute │ Value │
138 ├──────────┼───────────────┼────────────────────┤
139 │tzset() │ Thread safety │ MT-Safe env locale │
140 └──────────┴───────────────┴────────────────────┘
142 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
143
145 4.3BSD had a function char *timezone(zone, dst) that returned the name
146 of the timezone corresponding to its first argument (minutes West of
147 UTC). If the second argument was 0, the standard name was used, other‐
148 wise the daylight saving time version.
149
151 date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
152
154 This page is part of release 5.10 of the Linux man-pages project. A
155 description of the project, information about reporting bugs, and the
156 latest version of this page, can be found at
157 https://www.kernel.org/doc/man-pages/.
158 2017-09-15 TZSET(3)