man3/tzset.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  25 .\" References consulted:
  26 .\"     Linux libc source code
  27 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  28 .\"     386BSD man pages
  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>
  32 .\"
  33 .TH TZSET 3  2017-09-15 "" "Linux Programmer's Manual"
  34 .SH NAME
  35 tzset, tzname, timezone, daylight \- initialize time conversion information
  36 .SH SYNOPSIS
  37 .nf
  38 .B #include <time.h>
  39 .PP
  40 .B void tzset (void);
  41 .PP
  42 .BI "extern char *" tzname [2];
  43 .BI "extern long " timezone ;
  44 .BI "extern int " daylight ;
  45 .fi
  46 .PP
  47 .in -4n
  48 Feature Test Macro Requirements for glibc (see
  49 .BR feature_test_macros (7)):
  50 .in
  51 .PP
  52 .BR tzset ():
  53 _POSIX_C_SOURCE
  54 .br
  55 .IR tzname :
  56 _POSIX_C_SOURCE
  57 .br
  58 .IR timezone ,
  59 .IR daylight :
  60 _XOPEN_SOURCE
  61     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
  62     || /* Glibc versions <= 2.19: */ _SVID_SOURCE
  63 .SH DESCRIPTION
  64 The
  65 .BR tzset ()
  66 function initializes the \fItzname\fP variable from the
  67 .B TZ
  68 environment variable.
  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).
  75 .PP
  76 If the
  77 .B TZ
  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"
  81 .IR /etc/localtime .
  82 A timezone database of these files may be located in the system
  83 timezone directory (see the \fBFILES\fP section below).
  84 .PP
  85 If the
  86 .B TZ
  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.
  90 .PP
  91 The value of
  92 .B TZ
  93 can be one of two formats.
  94 The first format is a string of characters that directly represent the
  95 timezone to be used:
  96 .PP
  97 .in +4n
  98 .EX
  99 .IR "std offset" [ dst [ offset ][, start [ /time ], end [ /time ]]]
 100 .EE
 101 .in
 102 .PP
 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 (-)
 108 sign, and digits.
 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
 114 east.
 115 The hour must be between 0 and 24, and the minutes and seconds 00 and 59:
 116 .PP
 117 .in +4n
 118 .EX
 119 .RI [ + | - ] hh [ :mm [ :ss ]]
 120 .EE
 121 .in
 122 .PP
 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.
 127 .PP
 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
 130 standard time.
 131 These fields may have the following formats:
 132 .TP
 133 J\fIn\fP
 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.
 138 .TP
 139 .I n
 140 This specifies the zero-based Julian day with \fIn\fP between 0 and 365.
 141 February 29 is counted in leap years.
 142 .TP
 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).
 146 Week 1 is
 147 the first week in which day \fId\fP occurs and week 5 is the last week
 148 in which day \fId\fP occurs.
 149 Day 0 is a Sunday.
 150 .PP
 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.
 154 .PP
 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:
 160 .in +4n
 161 .EX
 162 TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"
 163 .EE
 164 .in
 165 .PP
 166 The second format specifies that the timezone information should be read
 167 from a file:
 168 .PP
 169 .in +4n
 170 .EX
 171 :[filespec]
 172 .EE
 173 .in
 174 .PP
 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.
 184 .PP
 185 Here's an example, once more for New Zealand:
 186 .PP
 187 .in +4n
 188 .EX
 189 TZ=":Pacific/Auckland"
 190 .EE
 191 .in
 192 .SH ENVIRONMENT
 193 .TP
 194 .B TZ
 195 If this variable is set its value takes precedence over the system
 196 configured timezone.
 197 .TP
 198 .B TZDIR
 199 If this variable is set its value takes precedence over the system
 200 configured timezone database directory path.
 201 .SH FILES
 202 .TP
 203 .I /etc/localtime
 204 The system timezone file.
 205 .TP
 206 .I /usr/share/zoneinfo/
 207 The system timezone database directory.
 208 .TP
 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.
 212 It is in the
 213 .BR tzfile "(5) format."
 214 By default, the zoneinfo Makefile hard links it to the
 215 .IR America/New_York " tzfile."
 216 .PP
 217 Above are the current standard file locations, but they are
 218 configurable when glibc is compiled.
 219 .SH ATTRIBUTES
 220 For an explanation of the terms used in this section, see
 221 .BR attributes (7).
 222 .TS
 223 allbox;
 224 lb lb lb
 225 l l l.
 226 Interface       Attribute       Value
 227 T{
 228 .BR tzset ()
 229 T}      Thread safety   MT-Safe env locale
 230 .TE
 231 .SH CONFORMING TO
 232 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 233 .SH NOTES
 234 .PP
 235 4.3BSD had a function
 236 .BI "char *timezone(" zone ", " dst )
 237 that returned the
 238 name of the timezone corresponding to its first argument (minutes
 239 West of UTC).
 240 If the second argument was 0, the standard name was used,
 241 otherwise the daylight saving time version.
 242 .SH SEE ALSO
 243 .BR date (1),
 244 .BR gettimeofday (2),
 245 .BR time (2),
 246 .BR ctime (3),
 247 .BR getenv (3),
 248 .BR tzfile (5)