Synchronize time on a private network that must not connect to the Internet
Solution 1:
If you have a convienent window nearby a GPS dongle can provide time information in addition to location.
Solution 2:
I definitely can recommend the Meinberg devices. We were using a 19" device which got it's time from DCF77 (Europe) in a large, but closed network (it was a closed production plant network with an Active Directory (don't ask..), so a proper time sync was inevitable for authentication/kerberos etc. The Mainberg device was rock solid, once set up, ran 'forever' - until the plant.. well ;-). Edit: I have no experience with the USB devices.