Time synchronization across dial-up / VPN

[Networker]

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.