xref: /dragonfly/lib/libc/gen/tzset.3 (revision 9f7604d7) 
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31.\" $FreeBSD: src/lib/libc/gen/tzset.3,v 1.6.2.5 2001/12/14 18:33:51 ru Exp $
32.\"
33.Dd October 19, 2008
34.Dt TZSET 3
35.Os
36.Sh NAME
37.Nm tzset ,
38.Nm tzsetwall
39.Nd initialize time conversion information
40.Sh LIBRARY
41.Lb libc
42.Sh SYNOPSIS
43.In time.h
44.Ft void
45.Fn tzset void
46.Ft void
47.Fn tzsetwall void
48.Sh DESCRIPTION
49The
50.Fn tzset
51function
52initializes time conversion information used by the library routine
53.Xr localtime 3 .
54The environment variable
55.Ev TZ
56specifies how this is done.
57.Pp
58If
59.Ev TZ
60does not appear in the environment, the best available approximation to
61local wall clock time, as specified by the
62.Xr tzfile 5 Ns -format
63file
64.Pa /etc/localtime
65is used.
66.Pp
67If
68.Ev TZ
69appears in the environment but its value is a null string, Coordinated
70Universal Time
71.Pq Tn UTC
72is used (without leap second correction).
73.Pp
74If
75.Ev TZ
76appears in the environment and its value begins with a colon
77.Pq Ql \&: ,
78the rest of its value is used as a pathname of a
79.Xr tzfile 5 Ns -format
80file from which to read the time conversion information.
81If the first character of the pathname is a slash
82.Pq Ql /
83it is used as
84an absolute pathname; otherwise, it is used as a pathname relative to
85the system time conversion information directory.
86.Pp
87If its value does not begin with a colon, it is first used as the pathname
88of a file (as described above) from which to read the time conversion
89information.
90If that file cannot be read, the value is then interpreted as a direct
91specification (the format is described below) of the time conversion
92information.
93.Pp
94If the
95.Ev TZ
96environment variable does not specify a
97.Xr tzfile 5 Ns -format
98file and cannot be interpreted as a direct specification,
99.Tn UTC
100is used.
101.Pp
102The
103.Fn tzsetwall
104function
105sets things up so that
106.Xr localtime 3
107returns the best available approximation of local wall clock time.
108.Sh SPECIFICATION FORMAT
109When
110.Ev TZ
111is used directly as a specification of the time conversion information,
112it must have the following syntax (spaces inserted for clarity):
113.Bd -ragged -offset indent
114.Em std offset
115.Bo
116.Em dst
117.Bq Em offset
118.Bq , Em rule
119.Bc
120.Ed
121.Pp
122Where:
123.Bl -tag -width std_and_dst -offset indent
124.It Em std No and Em dst
125Three or more bytes that are the designation for the standard
126.Pq Em std
127or summer
128.Pq Em dst
129time zone.  Only
130.Em std
131is required; if
132.Em dst
133is missing, then summer time does not apply in this locale.
134Upper and lowercase letters are explicitly allowed.  Any characters
135except a leading colon
136.Pq Ql \&: ,
137digits, comma
138.Pq Ql \&, ,
139minus
140.Pq Ql \- ,
141plus
142.Pq Ql + ,
143and
144.Tn ASCII
145.Dv NUL
146are allowed.
147.It Em offset
148Indicates the value one must add to the local time to arrive at
149Coordinated Universal Time.  The
150.Em offset
151has the form:
152.Bd -ragged -offset indent
153.Sm off
154.Em hh Bo
155.Em : mm
156.Bq Em : ss
157.Bc
158.Sm on
159.Ed
160.Pp
161The minutes
162.Pq Em mm
163and seconds
164.Pq Em ss
165are optional.  The hour
166.Pq Em hh
167is required and may be a single digit.  The
168.Em offset
169following
170.Em std
171is required.  If no
172.Em offset
173follows
174.Em dst ,
175summer time is assumed to be one hour ahead of standard time.  One or
176more digits may be used; the value is always interpreted as a decimal
177number.  The hour must be between zero and 24, and the minutes (and
178seconds) \(em if present \(em between zero and 59.  If preceded by a
179.Pq Ql \-
180the time zone shall be east of the Prime Meridian; otherwise it shall be
181west (which may be indicated by an optional preceding
182.Pq Ql + ) .
183.It Em rule
184Indicates when to change to and back from summer time.  The
185.Em rule
186has the form:
187.Bd -ragged -offset indent
188.Em date/time,date/time
189.Ed
190.Pp
191where the first
192.Em date
193describes when the change from standard to summer time occurs and the
194second
195.Em date
196describes when the change back happens.  Each
197.Em time
198field describes when, in current local time, the change to the other
199time is made.
200.Pp
201The format of
202.Em date
203is one of the following:
204.Bl -tag -width "M.m.n.d"
205.It Sy J Em n
206The Julian day
207.Em n
208(1 \*(Le
209.Em n
210\*(Le 365).
211Leap days are not counted; that is, in all years \(em including leap
212years \(em February 28 is day 59 and March 1 is day 60.  It is
213impossible to explicitly refer to the occasional February 29.
214.It Em n
215The zero-based Julian day
216(0 \*(Le
217.Em n
218\*(Le 365 ) .
219Leap days are counted, and it is possible to refer to February 29.
220.It Sy M  Em m.n.d
221The
222.Em d Ns 'th
223day (0 \*(Le
224.Em d
225\*(Le 6)
226of week
227.Em n
228of month
229.Em m
230of the year
231(1 \*(Le
232.Em n
233\*(Le 5),
234(1 \*(Le
235.Em m
236\*(Le 12),
237where week 5 means
238.Do
239the last
240.Em d
241day in month
242.Em m
243.Dc
244which may occur in either the fourth or the fifth week).  Week 1 is the
245first week in which the
246.Em d Ns 'th
247day occurs.  Day zero is Sunday.
248.Pp
249The
250.Em time
251has the same format as
252.Em offset
253except that no leading sign
254.Pq Ql \-
255or
256.Pq Ql +
257is allowed.  The default, if
258.Em time
259is not given, is
260.Sy 02:00:00 .
261.El
262.Pp
263If no
264.Em rule
265is present in the
266.Ev TZ
267specification, the rules specified
268by the
269.Xr tzfile 5 Ns -format
270file
271.Em posixrules
272in the system time conversion information directory are used, with the
273standard and summer time offsets from
274.Tn UTC
275replaced by those specified by
276the
277.Em offset
278values in
279.Ev TZ .
280.El
281.Pp
282For compatibility with System V Release 3.1, a semicolon
283.Pq Ql \&;
284may be used to separate the
285.Em rule
286from the rest of the specification.
287.Sh FILES
288.Bl -tag -width /usr/share/zoneinfo/posixrules -compact
289.It Pa /etc/localtime
290local time zone file
291.It Pa /usr/share/zoneinfo
292time zone directory
293.It Pa /usr/share/zoneinfo/posixrules
294rules for
295.Tn POSIX Ns -style
296.Tn TZ Ns 's
297.It Pa /usr/share/zoneinfo/GMT
298for
299.Tn UTC
300leap seconds
301.El
302.Pp
303If the file
304.Pa /usr/share/zoneinfo/GMT
305does not exist,
306.Tn UTC
307leap seconds are loaded from
308.Pa /usr/share/zoneinfo/posixrules .
309.Sh SEE ALSO
310.Xr date 1 ,
311.Xr gettimeofday 2 ,
312.Xr ctime 3 ,
313.Xr getenv 3 ,
314.Xr strftime 3 ,
315.Xr time 3 ,
316.Xr tzfile 5
317.Sh HISTORY
318The
319.Fn tzset
320and
321.Fn tzsetwall
322functions first appeared in
323.Bx 4.4 .
324