ADJTIME(2) System Calls ADJTIME(2)


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


#include <sys/time.h>

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


The adjtime() function adjusts the system's notion of the current time as
returned by gettimeofday(3C), advancing or retarding it by the amount of
time specified in the struct timeval pointed to by delta.

The adjustment is effected by speeding up (if that amount of time is
positive) or slowing down (if that amount of time is negative) the
system's clock by some small percentage, generally a fraction of one
percent. 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 delta is 0, then olddelta returns the status of the effects of the
previous adjtime() call with no effect on the time correction as a result
of this call. If olddelta is not a null pointer, then the structure it
points to will contain, upon successful return, the number of seconds
and/or microseconds still to be corrected from the earlier call. If
olddelta is a null pointer, the corresponding information will not be

This call may be used in 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.

Only a processes with appropriate privileges can adjust the time of day.

The adjustment value will be silently rounded to the resolution of the
system clock.


Upon successful completion, adjtime() returns 0. Otherwise, it returns -1
and sets errno to indicate the error.


The adjtime() function will fail if:

The delta or olddelta argument points outside the process's
allocated address space, or olddelta points to a region of the
process's allocated address space that is not writable.

The tv_usec member of delta is not within valid range (-1000000
to 1000000).

The {PRIV_SYS_TIME} privilege is not asserted in the effective
set of the calling process.

Additionally, the adjtime() function will fail for 32-bit interfaces if:

The size of the tv_sec member of the timeval structure
pointed to by olddelta is too small to contain the correct
number of seconds.


date(1), gettimeofday(3C), privileges(7)

September 19, 2022 ADJTIME(2)