1tzset(3) Library Functions Manual tzset(3)
2
6 tzset, tzname, timezone, daylight - initialize time conversion informa‐
7 tion
8
10 Standard C library (libc, -lc)
11
13 #include <time.h>
14 void tzset(void);
16 extern char *tzname[2];
18 extern long timezone;
19 extern int daylight;
20 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
22 tzset():
24 _POSIX_C_SOURCE
25 tzname:
27 _POSIX_C_SOURCE
28 timezone, daylight:
30 _XOPEN_SOURCE
31 || /* glibc >= 2.19: */ _DEFAULT_SOURCE
32 || /* glibc <= 2.19: */ _SVID_SOURCE
33
35 The tzset() function initializes the tzname variable from the TZ envi‐
36 ronment variable. This function is automatically called by the other
37 time conversion functions that depend on the timezone. In a System-V-
38 like environment, it will also set the variables timezone (seconds West
39 of UTC) and daylight (to 0 if this timezone does not have any daylight
40 saving time rules, or to nonzero if there is a time, past, present, or
41 future when daylight saving time applies).
42 If the TZ variable does not appear in the environment, the system time‐
44 zone is used. The system timezone is configured by copying, or link‐
45 ing, a file in the tzfile(5) format to /etc/localtime. A timezone
46 database of these files may be located in the system timezone directory
47 (see the FILES section below).
48 If the TZ variable does appear in the environment, but its value is
50 empty, or its value cannot be interpreted using any of the formats
51 specified below, then Coordinated Universal Time (UTC) is used.
52 The value of TZ can be one of two formats. The first format is a
54 string of characters that directly represent the timezone to be used:
55 std offset[dst[offset][,start[/time],end[/time]]]
57 There are no spaces in the specification. The std string specifies an
59 abbreviation for the timezone and must be three or more alphabetic
60 characters. When enclosed between the less-than (<) and greater-than
61 (>) signs, the character set is expanded to include the plus (+) sign,
62 the minus (-) sign, and digits. The offset string immediately follows
63 std and specifies the time value to be added to the local time to get
64 Coordinated Universal Time (UTC). The offset is positive if the local
65 timezone is west of the Prime Meridian and negative if it is east. The
66 hour must be between 0 and 24, and the minutes and seconds 00 and 59:
67 [+|-]hh[:mm[:ss]]
69 The dst string and offset specify the name and offset for the corre‐
71 sponding daylight saving timezone. If the offset is omitted, it de‐
72 faults to one hour ahead of standard time.
73 The start field specifies when daylight saving time goes into effect
75 and the end field specifies when the change is made back to standard
76 time. These fields may have the following formats:
77 Jn This specifies the Julian day with n between 1 and 365. Leap
79 days are not counted. In this format, February 29 can't be rep‐
80 resented; February 28 is day 59, and March 1 is always day 60.
81 n This specifies the zero-based Julian day with n between 0 and
83 365. February 29 is counted in leap years.
84 Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w <= 5) of
86 month m (1 <= m <= 12). Week 1 is the first week in which day d
87 occurs and week 5 is the last week in which day d occurs. Day 0
88 is a Sunday.
89 The time fields specify when, in the local time currently in effect,
91 the change to the other time occurs. If omitted, the default is
92 02:00:00.
93 Here is an example for New Zealand, where the standard time (NZST) is
95 12 hours ahead of UTC, and daylight saving time (NZDT), 13 hours ahead
96 of UTC, runs from the first Sunday in October to the third Sunday in
97 March, and the changeovers happen at the default time of 02:00:00:
98 TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
100 The second format specifies that the timezone information should be
102 read from a file:
103 :[filespec]
105 If the file specification filespec is omitted, or its value cannot be
107 interpreted, then Coordinated Universal Time (UTC) is used. If file‐
108 spec is given, it specifies another tzfile(5)-format file to read the
109 timezone information from. If filespec does not begin with a '/', the
110 file specification is relative to the system timezone directory. If
111 the colon is omitted each of the above TZ formats will be tried.
112 Here's an example, once more for New Zealand:
114 TZ=":Pacific/Auckland"
116
118 TZ If this variable is set its value takes precedence over the sys‐
119 tem configured timezone.
120 TZDIR If this variable is set its value takes precedence over the sys‐
122 tem configured timezone database directory path.
123
125 /etc/localtime
126 The system timezone file.
127 /usr/share/zoneinfo/
129 The system timezone database directory.
130 /usr/share/zoneinfo/posixrules
132 When a TZ string includes a dst timezone without anything fol‐
133 lowing it, then this file is used for the start/end rules. It
134 is in the tzfile(5) format. By default, the zoneinfo Makefile
135 hard links it to the America/New_York tzfile.
136 Above are the current standard file locations, but they are config‐
138 urable when glibc is compiled.
139
141 For an explanation of the terms used in this section, see at‐
142 tributes(7).
143 ┌─────────────────────────────────┬───────────────┬────────────────────┐
145 │Interface │ Attribute │ Value │
146 ├─────────────────────────────────┼───────────────┼────────────────────┤
147 │tzset() │ Thread safety │ MT-Safe env locale │
148 └─────────────────────────────────┴───────────────┴────────────────────┘
149
151 POSIX.1-2008.
152
154 POSIX.1-2001, SVr4, 4.3BSD.
155 4.3BSD had a function char *timezone(zone, dst) that returned the name
157 of the timezone corresponding to its first argument (minutes West of
158 UTC). If the second argument was 0, the standard name was used, other‐
159 wise the daylight saving time version.
160
162 date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
163Linux man-pages 6.04 2023-03-30 tzset(3)