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

> I've seen situations where people have put ntpdate in a cronjob to get
> around issues with big time jumps at boot or dodgy clocks under
> virtualization. There are much better solutions to this problem, so
> let us know if this is the case for you.
>

put "tinker panic 0" in your ntp.conf.

This stops ntpd from throwing a wobbly if it sees large jumps in time and
will keep clocks in virtualised environments ticking along.