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7534109d13
* Mention that mktime() and timegm() set errno on failure. * Correctly determining whether mktime() / timegm() succeeded with arbitrary input (where -1 can be a valid result) is non-trivial. Document the recommended procedure. PR: 277863 MFC after: 1 week Reviewed by: pauamma_gundo.com, gbe Differential Revision: https://reviews.freebsd.org/D44503
403 lines
9.5 KiB
Groff
403 lines
9.5 KiB
Groff
.\" Copyright (c) 1989, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" Arthur Olson.
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd March 26, 2024
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.Dt CTIME 3
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.Os
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.Sh NAME
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.Nm asctime ,
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.Nm asctime_r ,
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.Nm ctime ,
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.Nm ctime_r ,
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.Nm difftime ,
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.Nm gmtime ,
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.Nm gmtime_r ,
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.Nm localtime ,
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.Nm localtime_r ,
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.Nm mktime ,
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.Nm timegm
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.Nd transform binary date and time values
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In time.h
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.Vt extern char *tzname[2] ;
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.Ft char *
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.Fn asctime "const struct tm *tm"
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.Ft char *
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.Fn asctime_r "const struct tm *tm" "char *buf"
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.Ft char *
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.Fn ctime "const time_t *clock"
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.Ft char *
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.Fn ctime_r "const time_t *clock" "char *buf"
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.Ft double
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.Fn difftime "time_t time1" "time_t time0"
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.Ft struct tm *
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.Fn gmtime "const time_t *clock"
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.Ft struct tm *
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.Fn gmtime_r "const time_t *clock" "struct tm *result"
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.Ft struct tm *
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.Fn localtime "const time_t *clock"
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.Ft struct tm *
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.Fn localtime_r "const time_t *clock" "struct tm *result"
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.Ft time_t
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.Fn mktime "struct tm *tm"
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.Ft time_t
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.Fn timegm "struct tm *tm"
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.Sh DESCRIPTION
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The
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.Fn ctime ,
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.Fn gmtime ,
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and
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.Fn localtime
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functions all take as argument a pointer to a time value representing
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the time in seconds since the Epoch (00:00:00 UTC on January 1, 1970;
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see
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.Xr time 3 ) .
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.Pp
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The
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.Fn localtime
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function converts the time value pointed to by
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.Fa clock ,
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and returns a pointer to a
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.Vt struct tm
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(described below) which contains
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the broken-out time information for the value after adjusting for the current
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time zone (see
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.Xr tzset 3 ) .
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When the specified time translates to a year that will not fit in an
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.Vt int ,
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.Fn localtime
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returns
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.Dv NULL .
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The
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.Fn localtime
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function uses
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.Xr tzset 3
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to initialize time conversion information if
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.Xr tzset 3
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has not already been called by the process.
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.Pp
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After filling in the
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.Vt struct tm ,
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.Fn localtime
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sets the
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.Va tm_isdst Ns 'th
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element of
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.Va tzname
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to a pointer to an ASCII string that is the time zone abbreviation to be
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used with
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.Fn localtime Ns 's
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return value.
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.Pp
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The
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.Fn gmtime
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function similarly converts the time value, but without any time zone
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adjustment, and returns a pointer to a
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.Vt struct tm .
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.Pp
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The
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.Fn ctime
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function
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adjusts the time value for the current time zone in the same manner as
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.Fn localtime ,
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and returns a pointer to a 26-character string of the form:
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.Bd -literal -offset indent
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Thu Nov 24 18:22:48 1986\en\e0
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.Ed
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.Pp
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All the fields have constant width.
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.Pp
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The
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.Fn asctime
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function converts the broken down time in the
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.Vt struct tm
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pointed to by
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.Fa tm
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to the form shown in the example above.
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.Pp
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The
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.Fn ctime_r
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and
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.Fn asctime_r
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functions
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provide the same functionality as
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.Fn ctime
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and
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.Fn asctime
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except the caller must provide the output buffer
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.Fa buf ,
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which must be at least 26 characters long, to store the result in.
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The
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.Fn localtime_r
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and
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.Fn gmtime_r
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functions provide the same functionality as
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.Fn localtime
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and
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.Fn gmtime
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respectively, except the caller must provide the output buffer
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.Fa result .
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.Pp
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The functions
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.Fn mktime
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and
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.Fn timegm
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convert the broken-down time in the
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.Vt struct tm
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pointed to by
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.Fa tm
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into a time value with the same encoding as that of the values
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returned by the
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.Xr time 3
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function (that is, seconds from the Epoch, UTC).
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The
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.Fn mktime
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function interprets the input structure according to the current
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timezone setting (see
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.Xr tzset 3 )
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while the
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.Fn timegm
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function interprets the input structure as representing Universal
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Coordinated Time
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.Pq UTC .
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.Pp
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The original values of the
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.Fa tm_wday
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and
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.Fa tm_yday
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components of the structure are ignored, and the original values of the
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other components are not restricted to their normal ranges, and will be
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normalized if needed.
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For example,
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October 40 is changed into November 9,
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a
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.Fa tm_hour
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of \-1 means 1 hour before midnight,
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.Fa tm_mday
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of 0 means the day preceding the current month, and
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.Fa tm_mon
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of \-2 means 2 months before January of
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.Fa tm_year .
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(A positive or zero value for
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.Fa tm_isdst
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causes
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.Fn mktime
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to presume initially that summer time (for example, Daylight Saving Time)
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is or is not in effect for the specified time, respectively.
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A negative value for
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.Fa tm_isdst
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causes the
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.Fn mktime
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function to attempt to guess whether summer time is in effect for the
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specified time.
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The
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.Fa tm_isdst
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and
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.Fa tm_gmtoff
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members are forced to zero by
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.Fn timegm . )
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.Pp
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On successful completion, the values of the
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.Fa tm_wday
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and
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.Fa tm_yday
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components of the structure are set appropriately, and the other components
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are set to represent the specified calendar time, but with their values
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forced to their normal ranges; the final value of
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.Fa tm_mday
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is not set until
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.Fa tm_mon
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and
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.Fa tm_year
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are determined.
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The
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.Fn mktime
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function
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returns the specified calendar time; if the calendar time cannot be
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represented, it returns \-1 and sets
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.Xr errno 3
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to an appropriate value.
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.Pp
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Note that \-1 is a valid result (representing one second before
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midnight UTC on the evening of 31 December 1969), so this cannot be
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relied upon to indicate success or failure; instead,
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.Fa tm_wday
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and / or
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.Fa tm_yday
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should be set to an out-of-bounds value (e.g. \-1) prior to calling
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.Fn mktime
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or
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.Fn timegm
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and checked after the call.
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.Pp
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The
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.Fn difftime
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function returns the difference in seconds between two time values,
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.Fa time1
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\-
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.Fa time0 .
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.Pp
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External declarations as well as the definition of
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.Vt struct tm
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are in the
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.In time.h
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header.
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The
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.Vt tm
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structure includes at least the following fields:
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.Bd -literal -offset indent
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int tm_sec; /* seconds (0 - 60) */
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int tm_min; /* minutes (0 - 59) */
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int tm_hour; /* hours (0 - 23) */
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int tm_mday; /* day of month (1 - 31) */
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int tm_mon; /* month of year (0 - 11) */
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int tm_year; /* year \- 1900 */
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int tm_wday; /* day of week (Sunday = 0) */
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int tm_yday; /* day of year (0 - 365) */
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int tm_isdst; /* is summer time in effect? */
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char *tm_zone; /* abbreviation of timezone name */
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long tm_gmtoff; /* offset from UTC in seconds */
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.Ed
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.Pp
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The
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.Fa tm_isdst
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field is non-zero if summer time is in effect.
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.Pp
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The
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.Fa tm_gmtoff
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field is the offset in seconds of the time represented from UTC,
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with positive values indicating a time zone ahead of UTC (east of the
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Prime Meridian).
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.Sh SEE ALSO
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.Xr date 1 ,
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.Xr clock_gettime 2 ,
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.Xr gettimeofday 2 ,
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.Xr getenv 3 ,
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.Xr time 3 ,
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.Xr tzset 3 ,
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.Xr tzfile 5
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.Sh STANDARDS
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The
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.Fn asctime ,
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.Fn ctime ,
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.Fn difftime ,
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.Fn gmtime ,
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.Fn localtime ,
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and
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.Fn mktime
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functions conform to
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.St -isoC ,
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and conform to
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.St -p1003.1-96
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provided the selected local timezone does not contain a leap-second table
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(see
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.Xr zic 8 ) .
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.Pp
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The
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.Fn asctime_r ,
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.Fn ctime_r ,
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.Fn gmtime_r ,
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and
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.Fn localtime_r
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functions are expected to conform to
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.St -p1003.1-96
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(again provided the selected local timezone does not contain a leap-second
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table).
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.Pp
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The
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.Fn timegm
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function is not specified by any standard; its function cannot be
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completely emulated using the standard functions described above.
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.Sh HISTORY
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This manual page is derived from
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the time package contributed to Berkeley by
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.An Arthur Olson
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and which appeared in
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.Bx 4.3 .
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.Pp
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The functions
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.Fn asctime ,
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.Fn gmtime ,
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and
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.Fn localtime
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first appeared in
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.At v5 ,
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.Fn difftime
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and
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.Fn mktime
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in
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.Bx 4.3 Reno ,
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and
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.Fn timegm
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and
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.Fn timelocal
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in SunOS 4.0.
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.Pp
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The
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.Fn asctime_r ,
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.Fn ctime_r ,
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.Fn gmtime_r
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and
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.Fn localtime_r
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functions have been available since
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.Fx 8.0 .
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.Sh BUGS
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Except for
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.Fn difftime ,
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.Fn mktime ,
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and the
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.Fn \&_r
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variants of the other functions,
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these functions leave their result in an internal static object and return
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a pointer to that object.
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Subsequent calls to these
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function will modify the same object.
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.Pp
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The C Standard provides no mechanism for a program to modify its current
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local timezone setting, and the POSIX-standard
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method is not reentrant.
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(However, thread-safe implementations are provided
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in the POSIX threaded environment.)
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.Pp
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The
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.Va tm_zone
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field of a returned
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.Vt tm
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structure points to a static array of characters,
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which will also be overwritten by any subsequent calls (as well as by
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subsequent calls to
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.Xr tzset 3 ) .
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.Pp
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Use of the external variable
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.Fa tzname
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is discouraged; the
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.Fa tm_zone
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entry in the tm structure is preferred.
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