This article explains how to configure your Windows Server 2008 R2 Active Directory Primary Domain Controller to syncronise time with an external time source.
Note: In an active directory, all member computers will sync time with the PDC, so you only need to setup an external time source on the PDC.
Open an elevated command prompt and type the following:
Net Stop W32Time W32tm /config /syncfromflags:manual /manualpeerlist:"<ip addresses>" W32tm /config /reliable:yes Net Start W32Time W32tm /config /update W32tm /resync
You can run W32tm /resync on member computers to sync time with the PDC.
External Time Sources
In Australia, a good reliable time source to use is 0.au.pool.ntp.org.
Check out the NTP Project for world wide time sources.