.TH CTIME 3 
.SH NAME
ctime, localtime, gmtime, asctime, tzset, tzsetwall \- convert date and time to ASCII
.SH SYNOPSIS
.nf
.B extern char *tzname[2];
.PP
.B void tzset()
.PP
.B void tzsetwall()
.PP
.B char *ctime(clock)
.B long *clock;
.PP
.B #include <time.h>
.PP
.B char *asctime(tm)
.B struct tm *tm;
.PP
.B struct tm *localtime(clock)
.B long *clock;
.PP
.B struct tm *gmtime(clock)
.B long *clock;
.fi
.SH DESCRIPTION
.I Tzset
uses the value of the environment variable
.B TZ
to set time conversion information used by
.IR localtime .
If
.B TZ
does not appear in the environment,
the best available approximation to local wall clock time is used by
.IR localtime .
.if !\nX \{\
If
.B TZ
does appear in the environment,
.I localtime
will only work correctly if
its value is one of an
implementation-defined set of values.\}
.el \{\
If the value of
.B TZ
begins with a slash,
it is used as the absolute pathname of the
.IR tzfile (5)-format
file from which to read the time conversion information;
if
.I zonename
begins with some other character,
it is used as a pathname relative to a system time conversion information
directory.
\}
.PP
.I Tzsetwall
sets things up so that
.I localtime
returns the best available approximation of local wall clock time.
.PP
.I Ctime\^
converts a long integer, pointed to by
.IR clock ,
representing the time in seconds since
00:00:00 GMT, January 1, 1970,
and returns a pointer to a
26-character string
of the form
.br
.ce
Thu Nov 24 18:22:48 1986\\n\\0
.br
All the fields have constant width.
.PP
.IR Localtime\^
and
.I gmtime\^
return pointers to ``tm'' structures, described below.
.I Localtime\^
corrects for the time zone and any time zone adjustments
(Daylight Savings time in the U.S.A.);
before doing so,
.I localtime\^
calls
.I tzset\^
if
.I tzset\^
has not been called in the current process.
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 Greenwich Mean Time (GMT).
.PP
.I Asctime\^
converts a time value contained in a
``tm'' structure to a 26-character string,
as shown in the above example,
and returns a pointer
to the string.
.PP
Declarations of all the functions and externals, and the ``tm'' structure,
are in the
.B <time.h>\^
header file.
The structure declaration is:
.RS
.PP
.nf
struct tm {
        int tm_sec;	/\(** seconds (0 - 59) \(**/
        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 DST in effect? \(**/
};
.fi
.RE
.PP
.I Tm_isdst\^
is non-zero if a 
time zone adjustment such as Daylight Savings time
is in effect.
.SH SEE ALSO
.if \nX tzfile(5),
getenv(3),
time(2)
.SH BUGS
The return values point to static data
whose content is overwritten by each call.
.. %W%