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ADJTIME(2) System Calls Manual 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);

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 percent. 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 delta is NULL, no adjustment is done. If olddelta is non-NULL, the number of microseconds still to be corrected from the earlier call is stored into *olddelta. Setting the time with settimeofday(2) cancels any in-progress time adjustment.

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.

Only the superuser may adjust the time using the adjtime() function.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

adjtime() will fail if:
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Either of the arguments point outside the process's allocated address space.
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The delta argument is non-NULL and specifies a microsecond value less than zero or greater than or equal to one million.
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The delta argument is non-NULL and represents an adjustment greater than INT64_MAX microseconds or less than INT64_MIN microseconds.
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The delta argument is non-NULL and the process's effective user ID is not that of the superuser.

date(1), adjfreq(2), gettimeofday(2), ntpd(8)

The adjtime() function call appeared in 4.3BSD.

Other operating systems restrict calling adjtime() to the superuser and might not allow requesting the current correction without specifying a new value.
March 26, 2019 OpenBSD-current