Here is how I do this on a NT network.
On a single server (for you, the one with Internet access) I run a program called the JSS Clock Sync (free download at http://www.jarlsolidsolutions.se/)
This program will sync a Windows clock with an atomic clock once an hour. I just let it run on my server. It is on my PDC. For the following example, let's say my PDC's machine name is \\MYPDC.
Now, for my other servers, I have the following line in the logon script that my Administrative user account (the account I log into my servers with) uses:
NET TIME \\MYPDC /SET /Y
That syncs the server I am logging in with with \\MYPDC.
So, if you log into your systems once in a while, their clocks will be synchronize with the one server, and that oneserver is synchronize with a clock on the Internet.
hope this helps