Problem with clock and timer

The machine is actually a DMR-ES20, but was not on the list from above. The clock will not stay correct. It may drift to March 9, 2035. Setting the timer is futile. When I have set a program, it will record for four hours and lock up. Unplugging for a day will solve the problem temporarily. The same problems will occur. Is there reset button or a sequence of buttons to access a service mode? Is there a battery inside the machine that has gone dead? Got any ideas?