Hi guys, Thanks for your reply. I originally used the following command to synchronize time: ntpdate 192.168.0.191 but got the error: ntpdate[16715]: the NTP socket is in use, exiting So i used the way of stopping ntpd service. Now I have known that using command "ntpdate -u 192.168.0.191" can solve the above error, and also know using ntpdate command is an improper way, i.e. should use ntpd service instead of ntpdate command. But i have still a problem that why ntpdate command can cause backward clock? 192.168.0.191 is a valid ntpd server, its clock should not go backward, why the clock gotten by its client can go backwards?