Not sure if this will apply to your laptop, but it does apply to desktop PC's.
Most motherboards have a watch type battery on board, which can be replaced. This is responsible for keeping time, and on some older motherboards, for keeping the BIOS settings up to date. I think this must apply to laptops because otherwise time would be lost if you removed its power pack.
Consult your laptop manual/manufacturer to see if this applies. It's good news if it does - only costs a couple of quid to sort out.
Bodie