4.2BSD/usr/man/man2/gettimeofday.2

.TH GETTIMEOFDAY 2 "27 July 1983"
.UC 4
.SH NAME
gettimeofday, settimeofday \- get/set date and time
.SH SYNOPSIS
.nf
.ft B
#include <sys/time.h>
.PP
.ft B
gettimeofday(tp, tzp)
struct timeval *tp;
struct timezone *tzp;
.PP
.ft B
settimeofday(tp, tzp)
struct timeval *tp;
struct timezone *tzp;
.fi
.SH DESCRIPTION
.I Gettimeofday
returns the system's notion of the current Greenwich time and
the current time zone.  Time returned is expressed relative
in seconds and microseconds since midnight January 1, 1970.
.PP
The structures pointed to by
.I tp
and
.I tzp
are defined in 
.I <sys/time.h>
as:
.PP
.nf
.RS
.DT
struct timeval {
	u_long	tv_sec;		/* seconds since Jan. 1, 1970 */
	long	tv_usec;		/* and microseconds */
};
.sp 1
struct timezone {
	int	tz_minuteswest;	/* of Greenwich */
	int	tz_dsttime;	/* type of dst correction to apply */
};
.RE
.fi
.PP
The 
.I timezone
structure indicates the local time zone
(measured in minutes of time westward from Greenwich),
and a flag that, if nonzero, indicates that
Daylight Saving time applies locally during
the appropriate part of the year.
.PP
Only the super-user may set the time of day.
.SH RETURN
A 0 return value indicates that the call succeeded.
A \-1 return value indicates an error occurred, and in this
case an error code is stored into the global variable \fIerrno\fP.
.SH "ERRORS
The following error codes may be set in \fIerrno\fP:
.TP 15
[EFAULT]
An argument address referenced invalid memory.
.TP 15
[EPERM]
A user other than the super-user attempted to set the time.
.SH "SEE ALSO"
date(1), ctime(3)
.SH BUGS
Time is never correct enough to believe the microsecond
values.  There should a mechanism by which, at least,
local clusters of systems might synchronize their clocks
to millisecond granularity.