If it is a domain member, it will sync time from the Domain Controllers, which in turn sync from the server that holds the PDC Emulator role.
The server that holds the PDC Emulator FSMO role can have its time source set to sync from an external source with the following command:
w32tm /config /manualpeerlist:ServerToSyncFrom /syncfromflags:manual /reliable:yes /update
ServerToSyncFrom should be a reliable ntp time source, probably something from pool.ntp.org.
Then stop and restart the w32time service and you're good to go.
Edit: You should only do this on the PDC Emulator, though it will work on other servers. This will ensure that all time in the domain is in sync. If the clocks between the Kerberos KDC and the clients are out of sync by too much, any Kerberos tickets issues will be considered invalid (I believe 5 minutes is the threshold.) You don't want this to happen, because it will break SSO for file shares, printing and other nicely integrated magic that people take for granted.