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 >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
21 tzname: _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
22 timezone: _SVID_SOURCE || _XOPEN_SOURCE
23 daylight: _SVID_SOURCE || _XOPEN_SOURCE
24
26 The tzset() function initializes the tzname variable from the TZ envi‐
27 ronment variable. This function is automatically called by the other
28 time conversion functions that depend on the timezone. In a System-V-
29 like environment, it will also set the variables timezone (seconds West
30 of UTC) and daylight (to 0 if this timezone does not have any daylight
31 saving time rules, or to nonzero if there is a time during the year
32 when daylight saving time applies).
33 If the TZ variable does not appear in the environment, the tzname vari‐
35 able is initialized with the best approximation of local wall clock
36 time, as specified by the tzfile(5)-format file localtime found in the
37 system timezone directory (see below). (One also often sees
38 /etc/localtime used here, a symlink to the right file in the system
39 timezone directory.)
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 spec‐
43 ified below, Coordinated Universal Time (UTC) is used.
44 The value of TZ can be one of three formats. The first format is used
46 when there is no daylight saving time in the local timezone:
47 std offset
49 The std string specifies the name of the timezone and must be three or
51 more alphabetic characters. The offset string immediately follows std
52 and specifies the time value to be added to the local time to get Coor‐
53 dinated Universal Time (UTC). The offset is positive if the local
54 timezone is west of the Prime Meridian and negative if it is east. The
55 hour must be between 0 and 24, and the minutes and seconds 0 and 59.
56 The second format is used when there is daylight saving time:
58 std offset dst [offset],start[/time],end[/time]
60 There are no spaces in the specification. The initial std and offset
62 specify the standard timezone, as described above. The dst string and
63 offset specify the name and offset for the corresponding daylight sav‐
64 ing timezone. If the offset is omitted, it default to one hour ahead
65 of standard time.
66 The start field specifies when daylight saving time goes into effect
68 and the end field specifies when the change is made back to standard
69 time. These fields may have the following formats:
70 Jn This specifies the Julian day with n between 1 and 365. Febru‐
72 ary 29 is never counted even in leap years.
73 n This specifies the Julian day with n between 1 and 365. Febru‐
75 ary 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 third format specifies that the timezone information should be read
94 from a file:
95 :[filespec]
97 If the file specification filespec is omitted, the timezone information
99 is read from the file localtime in the system timezone directory, which
100 nowadays usually is /usr/share/zoneinfo. This file is in tzfile(5)
101 format. If filespec is given, it specifies another tzfile(5)-format
102 file to read the timezone information from. If filespec does not begin
103 with a '/', the file specification is relative to the system timezone
104 directory.
105 Here's an example, once more for New Zealand:
107 TZ=":Pacific/Auckland"
109
111 The system timezone directory used depends on the (g)libc version.
112 Libc4 and libc5 use /usr/lib/zoneinfo, and, since libc-5.4.6, when this
113 doesn't work, will try /usr/share/zoneinfo. Glibc2 will use the envi‐
114 ronment variable TZDIR, when that exists. Its default depends on how
115 it was installed, but normally is /usr/share/zoneinfo.
116 This timezone directory contains the files
118 localtime local timezone file
119 posixrules rules for POSIX-style TZ's
120 Often /etc/localtime is a symlink to the file localtime or to the cor‐
122 rect timezone file in the system timezone directory.
123
125 SVr4, POSIX.1-2001, 4.3BSD.
126
128 Note that the variable daylight does not indicate that daylight saving
129 time applies right now. It used to give the number of some algorithm
130 (see the variable tz_dsttime in gettimeofday(2)). It has been obsolete
131 for many years but is required by SUSv2.
132 4.3BSD had a function char *timezone(zone, dst) that returned the name
134 of the timezone corresponding to its first argument (minutes West of
135 UTC). If the second argument was 0, the standard name was used, other‐
136 wise the daylight saving time version.
137
139 date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
140
142 This page is part of release 3.25 of the Linux man-pages project. A
143 description of the project, and information about reporting bugs, can
144 be found at http://www.kernel.org/doc/man-pages/.
145 2010-02-25 TZSET(3)