Definitely replace the BIOS battery ASAP even if it takes some work!!
>>Is your computer old enough to need a board battery replacement (3-5 years)?
> 6 yrs in Dec
Definitely should be replaced, most probably the problem and if you leave it much longer it'll leak corrosive juice onto your motherboard.
If it's a laptop (sounds like it, with hard drive covering the BIOS battery), you should be able to open a hatch underneath and lift out the hard drive, get the manufacturer's pdf manuals from their website to suss this out (screws are sometimes under rubber feet which need to be removed & reglued). Go to Youtube and search on your computer's make and model with the word 'dissassemble' (also try 'dissassembly'). Unless it's a rare cheapo there's almost certainly a great guide there. Obviously put it on a second computer or fully study the video before starting laptop dissassembly!
Allocate lots of time for dissassembly, and if you'll be doing much of it, buy a jewellry organiser like this one for holding all the screws (hang it on a doorhandle or clothes rack with a hanger, put a little note with each set of screws in their pouch explaining exactly where they go for reassembly) and take lots of pics with your digicam or phone during the process so you're not left at the end with the proverbial few orphan screws.