PCI-10.4:

Obtain and review the process for getting and distributing the correct time within the organization. Also obtain and review related system parameter settings for the sample of (insert number and/or description of sample) system components. Verify the following is included in the process and implemented:

• NTP or similar technology is used for time synchronization.
• Two or three central time servers within the organization receive external time signals (directly from a special radio, GPS satellites, or other external sources—based on International Atomic Time and UTC (formerly GMT)), peer with each other to keep accurate time, and share the time with other internal servers (i.e., internal servers should not be all be receiving time signals from external sources).
• NTP is running the most recent version.
• Specific external hosts are designated from which the time servers will accept NTP time updates (to prevent an attacker from changing the clock). Optionally, those updates can be encrypted with a symmetric key, and access control lists can be created that specify the IP addresses of client machines that will be provided with the NTP service (to prevent unauthorized use of internal time servers).

See ntp.org for more information.