Ray
As others have mentioned, it's almost certainly your PRAM battery. This stores the date and time along with other things in a small mount of RAM until the computer is powered on again in which case it fetches this information from the PRAM Battery. If it's defaulting back to a default time, it does vary from machine to machine, but on my old Apple laptop it would be 00.00 1912, then it's almost certainly the PRAM battery.
Check online for your make and model and what battery you need as they do vary. Or if you can live with it (as I do) simply reset the date and time back to current and keep the laptop plugged and in sleep mode and trickle charging. The date and time won't change as long as the computer has a power source and isn't shut down.