How to configure an authoritative time server in Windows Server

All client desktop computers nominate the authenticating domain controller as their in-bound time partner.

All member servers follow the same process that client desktop computers follow.

All domain controllers in a domain nominate the primary domain controller (PDC) operations master as their in-bound time partner.

All PDC operations masters follow the hierarchy of domains in the selection of their in-bound time partner.

http://support.microsoft.com/kb/816042/en-us

http://www.nist.gov/pml/div688/grp40/its.cfm