Time synchronization across dial-up / VPN

[NetHappy]

Looking for the best way to do this efficiently.
We have a central office with a central PDC acting as a time server.
There are several remote sites each with a Win2003 DC that get their time from the HQ.
The sites are connected with dial-up / VPN, and network access often goes down between them and HQ or even local internet services.
How can we make sure we have good time sync for all sites? Clients are not an issue right now.

 

[Davetoo]

Point each DC at your remote sites to the same time server that your PDC emulator is pointing to.

I'm Certifiable, not cert-ified.
It just means my answers are from experience, not a book.

There are no more PDC's! There are DC's with FSMO roles!

 

[NetHappy]

If these DC's are part of a large domain, how would we make the external time server, for example time.windows.com, to take priority over the one designated as the authoritative time server for that domain? Currently the NtpServer setting points to time.windows.com.