Tip 130: Update/Change Atomic Clocks
I like keeping my computer's clock synchronized with an atomic clock.
But Windows XP does not list a large selection of such time servers in the Control Panel's Date/Time applet on the "Internet Time" tab.

Using the Registry Editor you can 'customize' your selection choices.
Look for the section as follows:

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\DateTime\Servers

Each entry of the time server listing is a string value, named with a single-digit number.
Once you are there right mouse click the right pane and select *new* string value:

Name Type Data
(Default) REG_SZ 1

1 REG_SZ clock.linuxshell.net
2 REG_SZ sundial.columbia.edu
3 REG_SZ timex.cs.columbia.edu
4 REG_SZ ntp.ctr.columbia.edu

Save the new entries.
Reboot your system and you will see the new addresses listed in the Control Panel Date/Time applet (on the "Internet Time" tab).

For more Time Servers check out: http://tycho.usno.navy.mil/ntp.html
 

Also see: Accurately setting your computer clock