Time Server The PDC emulator of a domain is authoritative for the domain time. The PDC emulator at the root of the forest becomes authoritative for the enterprise, and should be configured to gather the time from an external source. All PDC FSMO role holders follow the hierarchy of domains in the selection of their in-bound time partner. Here is how you sync your AD that has the PDC Role with external source so your clients/servers can always sync time through AD.

W32tm /config /syncfromflags:manual /manualpeerlist:"time.ucla.edu time2.ucla.edu time3.ucla.edu time4.ucla.edu" 
W32tm /config /reliable:yes
Net Start W32Time
W32tm /config /update
W32tm /resync

Please note to chanage time.ucla.edu time2.ucla.edu … to your own campus time server or one of nist free time server below:http://tf.nist.gov/tf-cgi/servers.cgi

Check Configuration:

w32tm /query /configuration

All Other DCs and Servers Run the following:

net stop w32Time
w32tm /unregister
w32tm /register 
net start w32time