Man page 2 : adjtime

adjtime - smoothly tune kernel clock

#include <sys/time.h>

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

This function speeds up or slows down the system clock in order to make a gradual adjustment. This ensures that the calendar time reported by the system clock is always monotonically increasing, which might not happen if you simply set the clock. The deltaargument specifies a relative adjustment to be made to the clock time. If negative, the system clock is slowed down fora while until it has lost this much elapsed time. If positive, the system clock is speeded up for a while. If the olddeltaargument is not a null pointer, the

adjtime function returns information about any previous time adjustment that has not yet completed. This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it.

The

adjtime function returns 0 on success and -1 on failure and sets the external variable errnoaccordingly.

EPERM

    The calling process does not have enough privileges to access the clock.

With a Linux kernel, you can use the

R adjtimex (2) function to permanently change the system clock speed.

This function is derived from 4.3 BSD and SVr4.

R adjtimex (2),

R settimeofday (2)