1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
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8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
17 .\" have taken the same level of care in the production of this manual,
18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" References consulted:
26 .\" Linux libc source code
27 .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
29 .\" Modified Sun Jul 25 11:01:58 1993 by Rik Faith (faith@cs.unc.edu)
30 .\" Modified 2001-11-13, aeb
31 .\" Modified 2004-12-01 mtk and Martin Schulze <joey@infodrom.org>
33 .TH TZSET 3 2017-09-15 "" "Linux Programmer's Manual"
35 tzset, tzname, timezone, daylight \- initialize time conversion information
42 .BI "extern char *" tzname [2];
43 .BI "extern long " timezone ;
44 .BI "extern int " daylight ;
48 Feature Test Macro Requirements for glibc (see
49 .BR feature_test_macros (7)):
61 || /* Glibc since 2.19: */ _DEFAULT_SOURCE
62 || /* Glibc versions <= 2.19: */ _SVID_SOURCE
66 function initializes the \fItzname\fP variable from the
69 This function is automatically called by the
70 other time conversion functions that depend on the timezone.
71 In a System-V-like environment, it will also set the variables \fItimezone\fP
72 (seconds West of UTC) and \fIdaylight\fP (to 0 if this timezone does not
73 have any daylight saving time rules, or to nonzero if there is a time,
74 past, present or future when daylight saving time applies).
78 variable does not appear in the environment, the system timezone is used.
79 The system timezone is configured by copying, or linking, a file in the
80 .BR tzfile "(5) format to"
82 A timezone database of these files may be located in the system
83 timezone directory (see the \fBFILES\fP section below).
87 variable does appear in the environment, but its value is empty,
88 or its value cannot be interpreted using any of the formats specified
89 below, then Coordinated Universal Time (UTC) is used.
93 can be one of two formats.
94 The first format is a string of characters that directly represent the
99 .IR "std offset" [ dst [ offset ][, start [ /time ], end [ /time ]]]
103 There are no spaces in the specification.
104 The \fIstd\fP string specifies an abbreviation for the timezone and must be
105 three or more alphabetic characters.
106 When enclosed between the less-than (<) and greater-than (>) signs, the
107 characters set is expanded to include the plus (+) sign, the minus (-)
109 The \fIoffset\fP string immediately
110 follows \fIstd\fP and specifies the time value to be added to the local
111 time to get Coordinated Universal Time (UTC).
112 The \fIoffset\fP is positive
113 if the local timezone is west of the Prime Meridian and negative if it is
115 The hour must be between 0 and 24, and the minutes and seconds 00 and 59:
119 .RI [ + | - ] hh [ :mm [ :ss ]]
123 The \fIdst\fP string and \fIoffset\fP specify the name and offset for the
124 corresponding daylight saving timezone.
125 If the offset is omitted,
126 it defaults to one hour ahead of standard time.
128 The \fIstart\fP field specifies when daylight saving time goes into
129 effect and the \fIend\fP field specifies when the change is made back to
131 These fields may have the following formats:
134 This specifies the Julian day with \fIn\fP between 1 and 365.
135 Leap days are not counted.
136 In this format, February 29 can't be represented;
137 February 28 is day 59, and March 1 is always day 60.
140 This specifies the zero-based Julian day with \fIn\fP between 0 and 365.
141 February 29 is counted in leap years.
143 M\fIm\fP.\fIw\fP.\fId\fP
144 This specifies day \fId\fP (0 <= \fId\fP <= 6) of week \fIw\fP
145 (1 <= \fIw\fP <= 5) of month \fIm\fP (1 <= \fIm\fP <= 12).
147 the first week in which day \fId\fP occurs and week 5 is the last week
148 in which day \fId\fP occurs.
151 The \fItime\fP fields specify when, in the local time currently in effect,
152 the change to the other time occurs.
153 If omitted, the default is 02:00:00.
155 Here is an example for New Zealand,
156 where the standard time (NZST) is 12 hours ahead of UTC,
157 and daylight saving time (NZDT), 13 hours ahead of UTC,
158 runs from the first Sunday in October to the third Sunday in March,
159 and the changeovers happen at the default time of 02:00:00:
162 TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
166 The second format specifies that the timezone information should be read
175 If the file specification \fIfilespec\fP is omitted, or its value cannot
176 be interpreted, then Coordinated Universal Time (UTC) is used.
177 If \fIfilespec\fP is given, it specifies another
178 .BR tzfile (5)-format
179 file to read the timezone information from.
180 If \fIfilespec\fP does not begin with a \(aq/\(aq, the file specification is
181 relative to the system timezone directory.
182 If the colon is omitted each
183 of the above \fBTZ\fP formats will be tried.
185 Here's an example, once more for New Zealand:
189 TZ=":Pacific/Auckland"
195 If this variable is set its value takes precedence over the system
199 If this variable is set its value takes precedence over the system
200 configured timezone database directory path.
204 The system timezone file.
206 .I /usr/share/zoneinfo/
207 The system timezone database directory.
209 .I /usr/share/zoneinfo/posixrules
210 When a TZ string includes a dst timezone without anything following it,
211 then this file is used for the start/end rules.
213 .BR tzfile "(5) format."
214 By default, the zoneinfo Makefile hard links it to the
215 .IR America/New_York " tzfile."
217 Above are the current standard file locations, but they are
218 configurable when glibc is compiled.
220 For an explanation of the terms used in this section, see
226 Interface Attribute Value
229 T} Thread safety MT-Safe env locale
232 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
235 4.3BSD had a function
236 .BI "char *timezone(" zone ", " dst )
238 name of the timezone corresponding to its first argument (minutes
240 If the second argument was 0, the standard name was used,
241 otherwise the daylight saving time version.
244 .BR gettimeofday (2),