On RHEL/CentOS you can force ntpd to syncronize on service startup by doing the following:
# vi /etc/sysconfig/ntpd
There is an OPTIONS variable with something like:
OPTIONS=”-u ntp:ntp -p /var/run/ntpd.pid”
Add the -x parameter:
OPTION=”-x -u ntp:ntp -p /var/run/ntpd.pid”
You may also want to set SYNC_HWCLOCK option to “yes”, in order to also syncronize hardware clock.
Then restart the service:
# service ntpd restart
The ntp daemon will be forced to resync every time you restart the service.