Windows Authenticated NTP?

Is there a way to use an authentication key when configuring Windows Time to point to a NTP Server that is not part of my domain? My situation is that I'm setting up a new Active Directory forest and I cannot find a way to use an authentication key when configuring the Windows Time service. I've read a couple articles about Active Directory clients using Kerberos to provide authentication within a domain, but what about the authoritative source in the domain (the PDC emulator)? Thanks in advance.