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

Another benefit to using NTP on Cisco PIX firewalls is found in a dynamic LAN to LAN VPN tunnel where the NTP request and response generates enough traffic to avoid the sometimes troublesome inactivity timeout.

Assuming that your NTP server (10.0.0.1) is accessible through the inside interface:

clock timezone EST -5
clock summer-time EDT recurring first Sunday April 2:00 last Sunday October 2:00 60
ntp server 10.0.0.1 source inside

You can then verify the NTP association with:

• CST -6
• CST -6
• CST -6

You can then verify the NTP association with:
show ntp association detail

You can also verify the PIX clock with:
show clock detail