Wait for the more knowledgeable to report but a new CMOS Battery would seem to be in order.
Easy to replace and only £1, or so, to buy, in almost any Supermarket.
About the size of a 10p piece on the motherboard, with a number like CM032. Don't take it out until you are ready to replace it.
Get the replacement > Take the old one out > Instal the new one and, with luck, that should be it.