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+.\" Copyright (c) 1989, 1991, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" This code is derived from software contributed to Berkeley by
+.\" Arthur Olson.
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"     From: @(#)ctime.3	8.1 (Berkeley) 6/4/93
+.\" $FreeBSD: src/lib/libc/stdtime/ctime.3,v 1.22 2004/07/02 23:52:12 ru Exp $
+.\"
+.Dd January 2, 1999
+.Dt CTIME 3
+.Os
+.Sh NAME
+.Nm asctime ,
+.Nm asctime_r ,
+.Nm ctime ,
+.Nm ctime_r ,
+.Nm difftime ,
+.Nm gmtime ,
+.Nm gmtime_r ,
+.Nm localtime ,
+.Nm localtime_r ,
+.Nm mktime ,
+.Nm timegm
+.Nd transform binary date and time values
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In time.h
+.Vt extern char *tzname[2] ;
+.Ft char *
+.Fn asctime "const struct tm *timeptr"
+.Ft char *
+.Fn asctime_r "const struct tm *restrict timeptr" "char *restrict buf"
+.Ft char *
+.Fn ctime "const time_t *clock"
+.Ft char *
+.Fn ctime_r "const time_t *clock" "char *buf"
+.Ft double
+.Fn difftime "time_t time1" "time_t time0"
+.Ft struct tm *
+.Fn gmtime "const time_t *clock"
+.Ft struct tm *
+.Fn gmtime_r "const time_t *clock" "struct tm *result"
+.Ft struct tm *
+.Fn localtime "const time_t *clock"
+.Ft struct tm *
+.Fn localtime_r "const time_t *clock" "struct tm *result"
+.Ft time_t
+.Fn mktime "struct tm *timeptr"
+.Ft time_t
+.Fn timegm "struct tm *timeptr"
+.Sh DESCRIPTION
+The functions
+.Fn ctime ,
+.Fn gmtime ,
+and
+.Fn localtime
+all take as an argument a time value representing the time in seconds since
+the Epoch (00:00:00
+.Tn UTC ,
+January 1, 1970; see
+.Xr time 3 ) .
+.Pp
+The function
+.Fn localtime
+converts the time value pointed at by
+.Fa clock .
+It returns a pointer to a
+.Dq Fa struct tm
+(described below), which contains
+the broken-out time information for the value after adjusting for the current
+time zone (and any other factors such as Daylight Saving Time).
+Time zone adjustments are performed as specified by the
+.Ev TZ
+environment variable (see
+.Xr tzset 3 ) .
+The function
+.Fn localtime
+uses
+.Xr tzset 3
+to initialize time conversion information, if
+.Xr tzset 3
+has not already been called by the process.
+.Pp
+After filling in the tm structure,
+.Fn localtime
+sets the
+.Fa tm_isdst Ns 'th
+element of
+.Fa tzname
+to a pointer to an
+.Tn ASCII
+string containing the time zone abbreviation to be
+used with
+.Fn localtime Ns 's
+return value.
+.Pp
+The function
+.Fn gmtime
+also converts the time value, but makes no time zone adjustment.
+It returns a pointer to a tm structure (described below).
+.Pp
+The
+.Fn ctime
+function
+adjusts the time value for the current time zone, in the same manner as
+.Fn localtime .
+It returns a pointer to a 26-character string of the form:
+.Bd -literal -offset indent
+Thu Nov 24 18:22:48 1986\en\e0
+.Ed
+.Pp
+All of the fields have constant width.
+.Pp
+The
+.Fn ctime_r
+function
+provides the same functionality as
+.Fn ctime ,
+except that the caller must provide the output buffer
+.Fa buf
+(which must be at least 26 characters long) to store the result.
+The
+.Fn localtime_r
+and
+.Fn gmtime_r
+functions
+provide the same functionality as
+.Fn localtime
+and
+.Fn gmtime ,
+respectively, except the caller must provide the output buffer
+.Fa result .
+.Pp
+The
+.Fn asctime
+function
+converts the broken-out time in the structure
+.Fa tm
+(pointed at by
+.Fa *timeptr )
+to the form
+shown in the example above.
+.Pp
+The
+.Fn asctime_r
+function
+provides the same functionality as
+.Fn asctime ,
+except that the caller provides the output buffer
+.Fa buf
+(which must be at least 26 characters long) to store the result.
+.Pp
+The functions
+.Fn mktime
+and
+.Fn timegm
+convert the broken-out time
+(in the structure pointed to by
+.Fa *timeptr )
+into a time value with the same encoding as that of the
+values returned by the
+.Xr time 3
+function (that is, seconds from the Epoch,
+.Tn UTC ) .
+The
+.Fn mktime
+function
+interprets the input structure according to the current timezone setting
+(see
+.Xr tzset 3 ) .
+The
+.Fn timegm
+function interprets the input structure
+as representing Universal Coordinated Time
+.Pq Tn UTC .
+.Pp
+The original values of the
+.Fa tm_wday
+and
+.Fa tm_yday
+components of the structure are ignored. The original values of the
+other components are not restricted to their normal ranges and will be
+normalized, if need be.
+For example,
+October 40 is changed into November 9,
+a
+.Fa tm_hour
+of \-1 means 1 hour before midnight,
+.Fa tm_mday
+of 0 means the day preceding the current month, and
+.Fa tm_mon
+of \-2 means 2 months before January of
+.Fa tm_year .
+(A positive or zero value for
+.Fa tm_isdst
+causes
+.Fn mktime
+to presume initially that summer time (for example, Daylight Saving Time)
+is or is not (respectively) in effect for the specified time.
+A negative value for
+.Fa tm_isdst
+causes the
+.Fn mktime
+function to attempt to divine whether summer time is in effect for the
+specified time.
+The
+.Fa tm_isdst
+and
+.Fa tm_gmtoff
+members are forced to zero by
+.Fn timegm . )
+.Pp
+On successful completion, the values of the
+.Fa tm_wday
+and
+.Fa tm_yday
+components of the structure are set appropriately, and the other components
+are set to represent the specified calendar time, but with their values
+forced to their normal ranges; the final value of
+.Fa tm_mday
+is not set until
+.Fa tm_mon
+and
+.Fa tm_year
+are determined.
+The
+.Fn mktime
+function
+returns the specified calendar time; if the calendar time cannot be
+represented, it returns \-1;
+.Pp
+The
+.Fn difftime
+function
+returns the difference between two calendar times,
+.Pf ( Fa time1
+-
+.Fa time0 ) ,
+expressed in seconds.
+.Pp
+External declarations, as well as the tm structure definition,
+are contained in the
+.In time.h
+include file.
+The tm structure includes at least the following fields:
+.Bd -literal -offset indent
+int tm_sec;	/\(** seconds (0 - 60) \(**/
+int tm_min;	/\(** minutes (0 - 59) \(**/
+int tm_hour;	/\(** hours (0 - 23) \(**/
+int tm_mday;	/\(** day of month (1 - 31) \(**/
+int tm_mon;	/\(** month of year (0 - 11) \(**/
+int tm_year;	/\(** year \- 1900 \(**/
+int tm_wday;	/\(** day of week (Sunday = 0) \(**/
+int tm_yday;	/\(** day of year (0 - 365) \(**/
+int tm_isdst;	/\(** is summer time in effect? \(**/
+char \(**tm_zone;	/\(** abbreviation of timezone name \(**/
+long tm_gmtoff;	/\(** offset from UTC in seconds \(**/
+.Ed
+.Pp
+The
+field
+.Fa tm_isdst
+is non-zero if summer (i.e., Daylight Saving) time is in effect.
+.Pp
+The field
+.Fa tm_gmtoff
+is the offset (in seconds) of the time represented from
+.Tn UTC ,
+with positive
+values indicating locations east of the Prime Meridian.
+.Sh SEE ALSO
+.Xr date 1 ,
+.Xr gettimeofday 2 ,
+.Xr getenv 3 ,
+.Xr time 3 ,
+.Xr tzset 3 ,
+.Xr tzfile 5
+.Sh STANDARDS
+The
+.Fn asctime ,
+.Fn ctime ,
+.Fn difftime ,
+.Fn gmtime ,
+.Fn localtime ,
+and
+.Fn mktime
+functions conform to
+.St -isoC ,
+and conform to
+.St -p1003.1-96
+provided the selected local timezone does not contain a leap-second table
+(see
+.Xr zic 8 ) .
+.Pp
+The
+.Fn asctime_r ,
+.Fn ctime_r ,
+.Fn gmtime_r ,
+and
+.Fn localtime_r
+functions are expected to conform to
+.St -p1003.1-96
+(again provided the selected local timezone does not contain a leap-second
+table).
+.Pp
+The
+.Fn timegm
+function is not specified by any standard; its function cannot be
+completely emulated using the standard functions described above.
+.Sh HISTORY
+This manual page is derived from
+the time package contributed to Berkeley by
+.An Arthur Olson
+and which appeared in
+.Bx 4.3 .
+.Sh BUGS
+Except for
+.Fn difftime ,
+.Fn mktime ,
+and the
+.Fn \&_r
+variants of the other functions,
+these functions leaves their result in an internal static object and return
+a pointer to that object.
+Subsequent calls to these
+function will modify the same object.
+.Pp
+The C Standard provides no mechanism for a program to modify its current
+local timezone setting, and the
+.Tn POSIX Ns No \&-standard
+method is not reentrant.
+(However, thread-safe implementations are provided
+in the
+.Tn POSIX
+threaded environment.)
+.Pp
+The
+.Va tm_zone
+field of a returned
+.Vt tm
+structure points to a static array of characters,
+which will also be overwritten by any subsequent calls (as well as by
+subsequent calls to
+.Xr tzset 3
+and
+.Xr tzsetwall 3 ) .
+.Pp
+Use of the external variable
+.Fa tzname
+is discouraged; the
+.Fa tm_zone
+entry in the tm structure is preferred.