Configuration of NTP is an often overlooked step in setting up a VMware ESX environment.  It is important to keep accurate time not only on the guests but also on the hosts.  In the 3.5 release of ESX you can configure NTP from the VI Client, but some older releases require configuration from the Service Console.  Here are the steps involved in NTP configuration from the Service Console.

My steps below assume the usage of the nano text editor however you could certainly choose a different text editor.  For help with using nano see the nano Command Manual or this quick Introduction.

Modify the NTP config file:
nano /etc/ntp.conf

Contents of ntp.conf should look like:
restrict 127.0.0.1
restrict default kod nomodify notrap
server tick.usno.navy.mil
server tock.usno.navy.mil
driftfile /var/lib/ntp/drift

Modify the NTP Step Tickers config file:
nano /etc/ntp/step-tickers

Contents of step-tickers should look like:
tick.usno.navy.mil
tock.usno.navy.mil

Enable NTP in the firewall
esxcfg-firewall --enableService ntpClient

Restart the NTP service
service ntpd restart

Set the NTP service to start automatically at system startup
chkconfig ntpd on