[CentOS] Why the command 'service ntpd stop' cause the time reversed?

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?