.TH NEWCTIME 3
.SH NAME
asctime, ctime, difftime, gmtime, localtime, mktime \- convert date and time to ASCII
.SH SYNOPSIS
.nf
.B extern char *tzname[2];
.PP
.B void tzset()
.PP
.B #include <sys/types.h>
.PP
.B char *ctime(clock)
.B const time_t *clock;
.PP
.B double difftime(time1, time0)
.B time_t time1;
.B time_t time0;
.PP
.B #include <time.h>
.PP
.B char *asctime(tm)
.B const struct tm *tm;
.PP
.B struct tm *localtime(clock)
.B const time_t *clock;
.PP
.B struct tm *gmtime(clock)
.B const time_t *clock;
.PP
.B time_t mktime(tm)
.B struct tm *tm;
.PP
.B cc ... -ltz
.fi
.SH DESCRIPTION
.I Ctime\^
converts a long integer, pointed to by
.IR clock ,
and returns a pointer to a
string of the form
.br
.ce
.eo
Thu Nov 24 18:22:48 1986\n\0
.br
.ec
Years requiring fewer than four characters are padded with leading zeroes.
For years longer than four characters, the string is of the form
.br
.ce
.eo
Thu Nov 24 18:22:48     81986\n\0
.ec
.br
with five spaces before the year.
These unusual formats are designed to make it less likely that older
software that expects exactly 26 bytes of output will mistakenly output
misleading values for out-of-range years.
.PP
The
.BI * clock
time stamp represents the time in seconds since 1970-01-01 00:00:00
Coordinated Universal Time (UTC).
The POSIX standard says that time stamps must be nonnegative
and must ignore leap seconds.
Many implementations extend POSIX by allowing negative time stamps,
and can therefore represent time stamps that predate the
introduction of UTC and are some other flavor of Universal Time (UT).
Some implementations support leap seconds, in contradiction to POSIX.
.PP
.I Localtime\^
and
.I gmtime\^
return pointers to ``tm'' structures, described below.
.I Localtime\^
corrects for the time zone and any time zone adjustments
(such as Daylight Saving Time in the United States).
After filling in the ``tm'' structure,
.I localtime
sets the
.BR tm_isdst 'th
element of
.B tzname
to a pointer to an
ASCII string that's the time zone abbreviation to be used with
.IR localtime 's
return value.
.PP
.I Gmtime\^
converts to Coordinated Universal Time.
.PP
.I Asctime\^
converts a time value contained in a
``tm'' structure to a string,
as shown in the above example,
and returns a pointer to the string.
.PP
.I Mktime\^
converts the broken-down time,
expressed as local time,
in the structure pointed to by
.I tm
into a calendar time value with the same encoding as that of the values
returned by the
.I time
function.
The original values of the
.B tm_wday
and
.B 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
.B tm_isdst
causes
.I 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
.B tm_isdst
causes the
.I mktime
function to attempt to divine whether summer time is in effect
for the specified time; in this case it does not use a consistent
rule and may give a different answer when later
presented with the same argument.)
On successful completion, the values of the
.B tm_wday
and
.B 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
.B tm_mday
is not set until
.B tm_mon
and
.B tm_year
are determined.
.I Mktime\^
returns the specified calendar time;
If the calendar time cannot be represented,
it returns
.BR -1 .
.PP
.I Difftime\^
returns the difference between two calendar times,
.RI ( time1
-
.IR time0 ),
expressed in seconds.
.PP
Declarations of all the functions and externals, and the ``tm'' structure,
are in the
.B <time.h>\^
header file.
The structure (of type)
.B struct tm
includes the following fields:
.RS
.PP
.nf
.ta .5i +\w'long tm_gmtoff;\0\0'u
	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 UT in seconds \(**/
.fi
.RE
.PP
The
.I tm_zone
and
.I 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
.I Tm_isdst\^
is non-zero if summer time is in effect.
.PP
.I Tm_gmtoff
is the offset (in seconds) of the time represented
from UT, with positive values indicating east
of the Prime Meridian.
The field's name is derived from Greenwich Mean Time, a precursor of UT.
.SH FILES
.ta \w'/usr/local/etc/zoneinfo/posixrules\0\0'u
/usr/local/etc/zoneinfo	time zone information directory
.br
/usr/local/etc/zoneinfo/localtime	local time zone file
.br
/usr/local/etc/zoneinfo/posixrules	used with POSIX-style TZ's
.br
/usr/local/etc/zoneinfo/GMT	for UTC leap seconds
.sp
If
.B /usr/local/etc/zoneinfo/GMT
is absent,
UTC leap seconds are loaded from
.BR /usr/local/etc/zoneinfo/posixrules .
.SH SEE ALSO
getenv(3),
newstrftime(3),
newtzset(3),
time(2),
tzfile(5)
.SH NOTES
The return values point to static data;
the data is overwritten by each call.
The
.B tm_zone
field of a returned
.B "struct tm"
points to a static array of characters, which
will also be overwritten at the next call
(and by calls to
.IR tzset ).
.PP
.I Asctime\^
and
.I ctime\^
behave strangely for years before 1000 or after 9999.
The 1989 and 1999 editions of the C Standard say
that years from \-99 through 999 are converted without
extra spaces, but this conflicts with longstanding
tradition and with this implementation.
Traditional implementations of these two functions are
restricted to years in the range 1900 through 2099.
To avoid this portability mess, new programs should use
.I strftime\^
instead.
.\" This file is in the public domain, so clarified as of
.\" 2009-05-17 by Arthur David Olson.