NANOSECOND
time_tick = NANOSECOND / hz;
if (u_nsec >= (NANOSECOND >> 1)) {
u_nsec -= NANOSECOND;
if (u_sec == pps_tf[0].tv_sec && v_nsec < NANOSECOND -
if (u_nsec > (NANOSECOND >> 1))
u_nsec -= NANOSECOND;
else if (u_nsec < -(NANOSECOND >> 1))
u_nsec += NANOSECOND;