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():
21 _POSIX_C_SOURCE
22 tzname:
24 _POSIX_C_SOURCE
25 timezone, daylight:
27 _XOPEN_SOURCE
28 || /* Glibc since 2.19: */ _DEFAULT_SOURCE
29 || /* Glibc <= 2.19: */ _SVID_SOURCE
30
32 The tzset() function initializes the tzname variable from the TZ envi‐
33 ronment variable. This function is automatically called by the other
34 time conversion functions that depend on the timezone. In a System-V-
35 like environment, it will also set the variables timezone (seconds West
36 of UTC) and daylight (to 0 if this timezone does not have any daylight
37 saving time rules, or to nonzero if there is a time, past, present, or
38 future when daylight saving time applies).
39 If the TZ variable does not appear in the environment, the system time‐
41 zone is used. The system timezone is configured by copying, or link‐
42 ing, a file in the tzfile(5) format to /etc/localtime. A timezone
43 database of these files may be located in the system timezone directory
44 (see the FILES section below).
45 If the TZ variable does appear in the environment, but its value is
47 empty, or its value cannot be interpreted using any of the formats
48 specified below, then Coordinated Universal Time (UTC) is used.
49 The value of TZ can be one of two formats. The first format is a
51 string of characters that directly represent the timezone to be used:
52 std offset[dst[offset][,start[/time],end[/time]]]
54 There are no spaces in the specification. The std string specifies an
56 abbreviation for the timezone and must be three or more alphabetic
57 characters. When enclosed between the less-than (<) and greater-than
58 (>) signs, the characters set is expanded to include the plus (+) sign,
59 the minus (-) sign, and digits. The offset string immediately follows
60 std and specifies the time value to be added to the local time to get
61 Coordinated Universal Time (UTC). The offset is positive if the local
62 timezone is west of the Prime Meridian and negative if it is east. The
63 hour must be between 0 and 24, and the minutes and seconds 00 and 59:
64 [+|-]hh[:mm[:ss]]
66 The dst string and offset specify the name and offset for the corre‐
68 sponding daylight saving timezone. If the offset is omitted, it de‐
69 faults to one hour ahead of standard time.
70 The start field specifies when daylight saving time goes into effect
72 and the end field specifies when the change is made back to standard
73 time. These fields may have the following formats:
74 Jn This specifies the Julian day with n between 1 and 365. Leap
76 days are not counted. In this format, February 29 can't be rep‐
77 resented; February 28 is day 59, and March 1 is always day 60.
78 n This specifies the zero-based Julian day with n between 0 and
80 365. February 29 is counted in leap years.
81 Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w <= 5) of
83 month m (1 <= m <= 12). Week 1 is the first week in which day d
84 occurs and week 5 is the last week in which day d occurs. Day 0
85 is a Sunday.
86 The time fields specify when, in the local time currently in effect,
88 the change to the other time occurs. If omitted, the default is
89 02:00:00.
90 Here is an example for New Zealand, where the standard time (NZST) is
92 12 hours ahead of UTC, and daylight saving time (NZDT), 13 hours ahead
93 of UTC, runs from the first Sunday in October to the third Sunday in
94 March, and the changeovers happen at the default time of 02:00:00:
95 TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
97 The second format specifies that the timezone information should be
99 read from a file:
100 :[filespec]
102 If the file specification filespec is omitted, or its value cannot be
104 interpreted, then Coordinated Universal Time (UTC) is used. If file‐
105 spec is given, it specifies another tzfile(5)-format file to read the
106 timezone information from. If filespec does not begin with a '/', the
107 file specification is relative to the system timezone directory. If
108 the colon is omitted each of the above TZ formats will be tried.
109 Here's an example, once more for New Zealand:
111 TZ=":Pacific/Auckland"
113
115 TZ If this variable is set its value takes precedence over the sys‐
116 tem configured timezone.
117 TZDIR If this variable is set its value takes precedence over the sys‐
119 tem configured timezone database directory path.
120
122 /etc/localtime
123 The system timezone file.
124 /usr/share/zoneinfo/
126 The system timezone database directory.
127 /usr/share/zoneinfo/posixrules
129 When a TZ string includes a dst timezone without anything fol‐
130 lowing it, then this file is used for the start/end rules. It
131 is in the tzfile(5) format. By default, the zoneinfo Makefile
132 hard links it to the America/New_York tzfile.
133 Above are the current standard file locations, but they are config‐
135 urable when glibc is compiled.
136
138 For an explanation of the terms used in this section, see at‐
139 tributes(7).
140 ┌─────────────────────────────────┬───────────────┬────────────────────┐
142 │Interface │ Attribute │ Value │
143 ├─────────────────────────────────┼───────────────┼────────────────────┤
144 │tzset() │ Thread safety │ MT-Safe env locale │
145 └─────────────────────────────────┴───────────────┴────────────────────┘
146
148 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
149
151 4.3BSD had a function char *timezone(zone, dst) that returned the name
152 of the timezone corresponding to its first argument (minutes West of
153 UTC). If the second argument was 0, the standard name was used, other‐
154 wise the daylight saving time version.
155
157 date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
158
160 This page is part of release 5.13 of the Linux man-pages project. A
161 description of the project, information about reporting bugs, and the
162 latest version of this page, can be found at
163 https://www.kernel.org/doc/man-pages/.
164 2021-03-22 TZSET(3)