If you are hosting DHCP from that server, you can set it up to hand out the IP of that server (or any other NTP source you would like) to the clients as the come online. For the server, check out this Technet KB article for more information:
http://support.microsoft.com/directory/article.asp?ID=KB;EN-US;Q216734&Go here to see what USNO Time Server you should use (if in the US, of course;)):
http://tycho.usno.navy.mil/ntp.htmlAnd then validate your time against this bad boy right here:
http://www.time.gov/This should pretty much keep you covered. With respect to the error, it *might* not be any sort of firewall issue, as I get intermittent failures as well when syncing within my domain or to an outside source. Once this is configured, you will probably be fine but you might see those errors occasionally anyway because of network traffic or NTP server overhead at the time of the request. If you want to see if the port is blocked, you can just try "net time <server IP> /set /y" at the command line and see if it will connect. If not, then you may very well be blocked.