It could need the battery on the motherboard replaced, easily done, if it's under warranty take it back to where you purchased it, if not remove the screws from the back, remove the small Ni battery from the motherboard, and have a look in dick smith electronics or somewhere like that to get a new battery. make sure you get exactly the same battery type, they usually have a code number stamped on them, pretty simple to do
Cheers
Mitch
