bsd man page for adjtime

ADJTIME(2)							System Calls Manual							ADJTIME(2)

NAME
       adjtime - correct the time to allow synchronization of the system clock

SYNOPSIS
       #include <sys/time.h>

       adjtime(delta, olddelta)
       struct timeval *delta;
       struct timeval *olddelta;

DESCRIPTION
       Adjtime	makes small adjustments to the system time, as returned by gettimeofday(2), advancing or retarding it by the time specified by the
       timeval delta.  If delta is negative, the clock is slowed down by incrementing it more slowly than normal until the correction is complete.
       If  delta is positive, a larger increment than normal is used.  The skew used to perform the correction is generally a fraction of one per-
       cent.  Thus, the time is always a monotonically increasing function.  A time correction from an earlier call to adjtime may not be finished
       when adjtime is called again.  If olddelta is non-zero, then the structure pointed to will contain, upon return, the number of microseconds
       still to be corrected from the earlier call.

       This call may be used by time servers that synchronize the clocks of computers in a local area network.	Such time servers would slow  down
       the clocks of some machines and speed up the clocks of others to bring them to the average network time.

       The call adjtime(2) is restricted to the super-user.

RETURN VALUE
       A  return  value  of 0 indicates that the call succeeded.  A return value of -1 indicates that an error occurred, and in this case an error
       code is stored in the global variable errno.

ERRORS
       The following error codes may be set in errno:

       [EFAULT]       An argument points outside the process's allocated address space.

       [EPERM]	      The process's effective user ID is not that of the super-user.

SEE ALSO
       date(1), gettimeofday(2), timed(8), timedc(8),
       TSP: The Time Synchronization Protocol for UNIX 4.3BSD, R. Gusella and S. Zatti

NOTES  (PDP-11)
       Adjtime(2) calls are executed immediately, not over a period of time, therefore, the olddelta return values for	an  adjtime(2)	call  will
       always be zero.

4.3 Berkeley Distribution					   May 15, 1986 							ADJTIME(2)