.\"	$OpenBSD: ctime.3,v 1.16 1999/11/15 07:45:57 deraadt Exp $
.\"
.\"
.Dd February 16, 1999
.Dt CTIME 3
.Os
.Sh NAME
.Nm asctime ,
.Nm ctime ,
.N difftime ,
.Nm gmtime ,
.Nm localtime ,
.Nm mktime
.Nd convert date and time to ASCII
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <time.h>
.Ft extern char *tzname[2];
.Ft void
.Fn tzset "void"
.Ft "char *"
.Fn ctime "time_t *clock"
.Ft double
.Fn difftime "time_t time1" "time_t time0"
.Ft "char *"
.Fn asctime "const struct tm *tm"
.Ft "struct tm *"
.Fn localtime "const time_t *clock"
.Ft "struct tm *"
.Fn gmtime "const time_t *clock"
.Ft time_t
.Fn mktime "struct tm *tm"
.Sh DESCRIPTION
.Fn ctime
converts a
.Ft time_t ,
pointed to by
.Ft clock ,
representing the time in seconds since
00:00:00 UTC, 1970-01-01,
and returns a pointer to a
26-character string
of the form
.Bd -literal -offset indent
Thu Nov 24 18:22:48 1986\en\0
.Ed
.Pp
All the fields have constant width.
.Pp
.Fn localtime
and
.Fn gmtime
return pointers to
.Ft tm
structures, described below.
.Fn localtime
corrects for the time zone and any time zone adjustments
(such as Daylight Saving Time in the U.S.A.).
After filling in the
.Ft tm
structure,
.Fn localtime
sets the
.Ft tm_isdst
'th
element of
.Ft tzname
to a pointer to an
ASCII string that's the time zone abbreviation to be used with
the return value of
.Fn localtime .
.Pp
.Fn gmtime
converts to Coordinated Universal Time.
.Pp
.Fn asctime
converts a time value contained in a
.Ft tm
structure to a 26-character string,
as shown in the above example,
and returns a pointer
to the string.
.Pp
.Fn mktime
converts the broken-down time,
expressed as local time,
in the structure pointed to by
.Ft tm
into a calendar time value with the same encoding as that of the values
returned by the
.Fn time
function.
The original values of the
.Ft tm_wday
and
.Ft tm_yday
components of the structure are ignored,
and the original values of the other components are not restricted
to their normal ranges.
(A positive or zero value for
.Ft tm_isdst
causes
.Fn mktime
to presume initially that summer time (for example, Daylight Saving Time
in the U.S.A.)
respectively,
is or is not in effect for the specified time.
A negative value for
.Ft tm_isdst
causes the
.Fn mktime
function to attempt to divine whether summer time is in effect
for the specified time.)
On successful completion, the values of the
.Ft tm_wday
and
.Ft 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
.Ft tm_mday
is not set until
.Ft tm_mon
and
.Ft tm_year
are determined.
.Fn mktime
returns the specified calendar time;
If the calendar time cannot be represented,
it returns \-1.
.Pp
.Fn difftime
returns the difference between two calendar times,
(
.Ft time1
-
.Ft time0
), expressed in seconds.
.Pp
Declarations of all the functions and externals, and the
.Ft tm
structure, are in the
.Ft <time.h>
header file.
The structure (of type)
.Ft struct tm
includes 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
.Ft tm_zone
and
.Ft tm_gmtoff
fields exist, and are filled in, only if arrangements to do
so were made when the library containing these functions was
created.
There is no guarantee that these fields will continue to exist
in this form in future releases of this code.
.Pp
.Ft tm_isdst
is non-zero if summer time is in effect.
.Pp
.Ft tm_gmtoff
is the offset (in seconds) of the time represented
from UTC, with positive values indicating east
of the Prime Meridian.
.Sh FILES
.Bl -tag -width "/usr/share/zoneinfo/posixrules"
.It Pa /usr/share/zoneinfo
time zone information directory
.It Pa /etc/localtime
local time zone file
.It Pa /usr/share/zoneinfo/posixrules
used with POSIX-style TZ's
.It Pa /usr/share/zoneinfo/GMT
for UTC leap seconds
.El
.Pp
If
.Pa /usr/share/zoneinfo/GMT
is absent,
UTC leap seconds are loaded from
.Pa /usr/share/zoneinfo/posixrules .
.Sh SEE ALSO
.Xr getenv 3 ,
.Xr strftime 3 ,
.Xr time 3 ,
.Xr tzset 3 ,
.Xr tzfil 5
.Sh NOTES
The return values point to static data;
the data is overwritten by each call.
The
.Ft tm_zone
field of a returned
.Ft "struct tm"
points to a static array of characters, which
will also be overwritten at the next call
(and by calls to
.Fn tzset
).
.Pp
Avoid using out-of-range values with
.Fn mktime
when setting up lunch with promptness sticklers in Riyadh.
.\" based on @(#)newctime.3	7.13