Computer problems with the clock and date,what is powering these facets?

Currently running a Sony PCV-RS 630G I do believe there is a battery that powers the clock, where can I find this battery for replacement? PROBLEM I have to reset the clock about every 4 days, then it will loose 2 hours and upsets all of my times downloads and programs that are programed. Removed the cover to looked for a small battery but no success. Or am I looking for something non existent, if so what powers the clock and date on this unit.