http://jackstromberg.com/2013/10/configuring-external-time-source-on-your-primary-domain-controller/

Start a PowerShell Administrative Session, Then...

Get Name of PDC:
[System.DirectoryServices.ActiveDirectory.Forest]::GetCurrentForest().RootDomain.PdcRoleOwner.Name

Configure External Time Source:
w32tm.exe /config /manualpeerlist:”0.us.pool.ntp.org 1.us.pool.ntp.org 2.us.pool.ntp.org 3.us.pool.ntp.org” /syncfromflags:manual /reliable:YES /update

Update Time:
w32tm.exe /config /update

Restart Time Service:
Restart-Service w32time