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Note: There is a strain on the first tier of time servers. If you don't have a very specific need to be more accurate than 10ths of seconds, you should use your ISP's time server, something near you, or a random server from www.pool.ntp.com.
Setup the service to start at boot.
chkconfig ntpd on
Set the time server.
ntpdate ntp.xmission.com
Start the NTPD service (Redhat / CentOS way).
service ntpd start
Setup the clock to sync at boot, add the following to /etc/rc.local. This is not typically necessary, because the clock is set at regular intervals by the ntp daemon. But, on a system that has a time issue such as a dead CMOS battery, this can be useful.
# initial clock sync /etc/init.d/ntpd stop /usr/sbin/ntpdate -b ntp1.hurdman.org /usr/sbin/ntpdate -b ntp2.hurdman.org /etc/init.d/ntpd start