DIAGNOSIS The internal backup battery on the main control board is dead. Its sole job is to maintain clock memory during brief power outages.
FIX There is no user-replaceable battery. This is a known design trait of this model. The clock is not a critical function. You have two options: live with it, or leave the microwave plugged into a dedicated outlet 24/7 to maintain the time.
IF THAT DIDN'T WORK If the clock resets even while the unit remains continuously plugged in, the control board itself is failing and losing memory. This doesn't affect cooking functions.
HEADS UP Don't attempt to open the unit to replace a non-existent user battery. The clock reset is a minor inconvenience, not a symptom of a larger problem with the heating components.