TIME LOSS ON SONY 6000 CD PLAYER?

The clock on my 2008 Ford C-Max tends to lose not just minutes, but hours when the car is started!
Also the date can lose a couple of months overnight.
Despite resetting the clock it seems to be a recurring fault.
Anyone got any ideas about this fault?
I know I could take it to a Dealer, but they require at least a day to investigate!