Some mobile operators provide time/date information via the radio link. This may be the cause. It could be useful to check wether other models have the same issue.
On the other hand, I used to synchronize my N900 with this command (only when I had taken the battery apart):

rdate -s time.nist.gov

It must be run as root, BTW. Also, if you use alarmed (only for N900) you can run it periodically.

Hope this helps.
Regards.