I installed a program called ntpdate, and it connects to an NTP server on the internet when I am online, and updates the clock on my N800 automatically. http://maemo.org/downloads/product/OS2006/ntpdate/
I did a search and haven't seen this posted before.
You can get http://nitapps.com/repository.install and then be able to install gps_clockd which when the GPS is activated will sync the clock with the GPS satellites.
ntpdate doesn't do this automagically. You have to issue the command manually, and the command is superseeded by rdate.
If you want to do this automagically simply install openntpd. This is a very lightweight, secure ntpd by OpenBSD development team. Its available in Extras/.
The clock on my N800 is extremely accurate - many orders of magnitude better than any PC clock I've seen. So I synchronize with rdate occasionally (it could be a month or two between, or more), the clock is so accurate I don't bother with automatic update (unlike with my other computers).