I used to use the atomic clock and ping every now and then for accurate time, but since my XP install, I am informed that Port 13 access is blocked (behind the firewall) and that I should ask the administrator/proxy server to unblock it. You know what I am going to ask - how on earth do you set about doing this? How do I find out who this mysterious person/machine is? I wonder why the access has been blocked - it is apparently the port through which the time information is sent.

This may be of some help.
When I perform a time check, why do I receive an error message that says my connection is blocked (or unknown error 10050)?
The most common reason for connection problems is using AtomTime behind firewall software or a proxy server. Most companies/corporate environments uses a firewall/proxy server to protect their local network from outside intruders. In addition, many home users now run firewall software like ZoneAlarm. By default, a lot of these firewalls block TCP port 13, the port AtomTime uses to receive the time value from the time server.
Some steps to try to resolve the connection problems:

If using a program like ZoneAlarm, check that software and have it allow AtomTime connections.
Fill in the Proxy Server settings in AtomTime. Your network adminstrator should be able to help here, otherwise look in your web browser settings for the correct proxy server settings, specifically the settings for HTTP.
If you use Internet Explorer (not Netscape, it won't work for this test), try this exact URL: http://time-a.nist.gov:13/
It should return a timestamp. If it does not, talk to your network adminstrator and show them that URL. Explain that you'd like TCP port 13 opened so that the time value can be retrieved.
Try defining your own set of time servers, using port 37 as the time port.
Regards,
Tin