Have you done all your MS Updates?

Hi Marcus:

You're problem does not sound like an indication of something more serious.  
Remember that batteries are every once in a while bad-- even fresh out of the
box.  This is especially true if your battery is an odd type and has been
sitting in the store for awhile.  Pull out the "new" battery and have it
tested.  You do not mention how old your machine is.  It could also be a
loose connection between the battery and the thing-a-flotchy.  If that is the
case, and your unit is old, it is probably not worth fixing.  Unless you need
strict and precise time-stamping, you could add a command to the startup--
for convenience--  that would prompt you to enter the date at boot-up.

Other than that you may want to post in a hardware forum here or elsewhere.

Good Luck

Mark