The clock_settime function sets the specified clock, clock_id,
    to the value specified by tp. Time values that are between two
    consecutive non-negative integer multiples of the resolution of
    the specified clock are truncated down to the smaller multiple of
    the resolution.

    A clock can be systemwide (that is, visible to all processes)
    or per-process (measuring time that is meaningful only within a
    process).

    The CLOCK_REALTIME clock, defined in <time.h>, represents the
    realtime clock for the system. For this clock, the values
    specified by clock_settime and returned by clock_gettime
    represent the amount of time elapsed, in seconds and nanoseconds,
    since the Epoch. The Epoch is defined as 00:00:00:00 January 1,
    1970 Greenwich Mean Time (GMT).

    You must have OPER, LOG_IO, and SYSPRV privileges to use the
    clock_settime function.

    On success, the function returns 0.

    On failure, the function returns -1 and sets errno to indicate
    the error.

    See also clock_getres, clock_gettime, time, and ctime.