Time won't sync with NTP server on linux

I am throwing this one out there as I have seen it bite a few people in the past. You realize your system time is way off and setup ntp and get it running but the system time never updates... You have verified the config and ntp is setup properly. The most likely cause is the fact that ntpd won't correct massive clock skew. If you are off by a great deal you will have to get it in sync manually then ntpd will keep it in sync going forward. This is how you would deal with a clock skew on centos:

yum install ntp
chkconfig ntpd on
ntpdate 0.rhel.pool.ntp.org
/etc/init.d/ntpd start