To configure an external time server source for your Server 2008 R2 PDC Server, you can use following commands:
net stop w32time — Stop time server service
w32tm /config /syncfromflags:manual /manualpeerlist:"xxx.xxx.com" — Configure an external source
w32tm /config /reliable:yes — Make your pdc reliable for domain clients
net start w32time — Start time server service