.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 19:49:27 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Fri Apr 26 12:38:55 MET DST 1996 by Martin Schulze (joey@linux.de)
.\"
.TH CTIME 3  "April 26, 1996" "BSD" "Linux Programmer's Manual"
.SH NAME
asctime, ctime, gmtime, localtime, mktime \- transform binary date and time
to ASCII
.SH SYNOPSIS
.nf
.B #include <time.h>
.sp
.BI "char *asctime(const struct tm *" timeptr );
.sp
.BI "char *ctime(const time_t *" timep );
.sp
.BI "struct tm *gmtime(const time_t *" timep );
.sp
.BI "struct tm *localtime(const time_t *" timep );
.sp
.BI "time_t mktime(struct tm *" timeptr );
.sp
.BI "extern char *" tzname [2];
.BI "long int " timezone ;
.BI "extern int " daylight ;
.fi
.SH DESCRIPTION
The \fBctime()\fP, \fBgmtime()\fP and \fBlocaltime()\fP functions all take
an argument of data type \fItime_t\fP which represents calendar time.
When interpreted as an absolute time value, it represents the number of
seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal
Time (UTC).
.PP
The \fBasctime()\fP and \fBmktime()\fP functions both take an argument
representing broken-down time which is a binary representation
separated into year, month, day, etc.  Broken-down time is stored
in the structure \fItm\fP which is defined in \fI<time.h>\fP as follows:
.sp
.RS
.nf
.ne 12
.ta 8n 16n 32n
struct tm
{
	int	tm_sec;			/* seconds */
	int	tm_min;			/* minutes */
	int	tm_hour;		/* hours */
	int	tm_mday;		/* day of the month */
	int	tm_mon;			/* month */
	int	tm_year;		/* year */
	int	tm_wday;		/* day of the week */
	int	tm_yday;		/* day in the year */
	int	tm_isdst;		/* daylight saving time */
};
.ta
.fi
.RE
.PP
The members of the \fItm\fP structure are:
.TP
.I tm_sec
The number of seconds after the minute, normally in the range 0 to 59, 
but can be up to 61 to allow for leap seconds.
.TP
.I tm_min
The number of minutes after the hour, in the range 0 to 59. 
.TP
.I tm_hour
The number of hours past midnight, in the range 0 to 23.
.TP
.I tm_mday
The day of the month, in the range 1 to 31.
.TP
.I tm_mon
The number of months since January, in the range 0 to 11.
.TP
.I tm_year
The number of years since 1900.
.TP
.I tm_wday
The number of days since Sunday, in the range 0 to 6.
.TP
.I tm_yday
The number of days since January 1, in the range 0 to 365.
.TP
.I tm_isdst
A flag that indicates whether daylight saving time is in effect at the
time described.  The value is positive if daylight saving time is in
effect, zero if it is not, and negative if the information is not
available.
.PP
The \fBctime()\fP function converts the calendar time \fItimep\fP into a
string of the form
.sp
.RS
"Wed Jun 30 21:49:08 1993\\n"
.RE
.sp
The abbreviations for the days of the week are `Sun', `Mon', `Tue', `Wed',
`Thu', `Fri', and `Sat'.  The abbreviations for the months are `Jan',
`Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and
`Dec'.  The return value points to a statically allocated string which
might be overwritten by subsequent calls to any of the date and time
functions.  The function also sets the external variable \fItzname\fP
with information about the current time zone.
.PP
The \fBgmtime()\fP function converts the calendar time \fItimep\fP to
broken-down time representation, expressed in Coordinated Universal Time
(UTC).
.PP
The \fBlocaltime()\fP function converts the calendar time \fItimep\fP to
broken-time representation, expressed relative to the user's specified
time zone.    The function sets the external variables \fItzname\fP with 
information about the current time zone, \fItimezone\fP with the difference
between Coordinated Universal Time (UTC) and local standard time in
seconds, and \fIdaylight\fP to a non-zero value if standard US daylight
savings time rules apply.
.PP
The \fBasctime()\fP function converts the broken-down time value
\fItimeptr\fP into a string with the same format as \fBctime()\fP.
The return value points to a statically allocated string which might be 
overwritten by subsequent calls to any of the date and time functions.
.PP
The \fBmktime()\fP function converts a broken-down time structure, expressed
as local time, to
calendar time representation.  The function ignores the specified contents
of the structure members \fItm_wday\fP and \fItm_yday\fP and recomputes
them from the other information in the broken-down time structure.
Calling \fBmktime()\fP also sets the external variable \fItzname\fP with
information about the current time zone.  If the specified broken-down
time cannot be represented as calendar time, \fBmktime()\fP returns a
value of (time_t)(\-1) and does not alter the \fItm_wday\fP and \fItm_yday\fP
members of the broken-down time structure.
.SH "CONFORMING TO"
SVID 3, POSIX, BSD 4.3, ISO 9899
.SH "SEE ALSO"
.BR date "(1), " gettimeofday "(2), " time "(2), " tzset (3),
.BR difftime (3),
.BR strftime (3),
.BR newctime (3).