DIAGNOSIS Yes, the internal backup battery that holds the clock and settings is dead. It's a common issue after a few years.
FIX This requires opening the base. You'll need to unplug the machine, flip it over, remove the rubber feet to access the screws, and open the baseplate. The battery is a standard 3V CR2032 coin cell soldered to the main board. You'll need basic soldering skills to desolder the old one and solder in a new one.
IF THAT DIDN'T WORK If replacing the battery doesn't solve it, the main control board itself may have a fault, and you'd be looking at a board replacement, which is a more involved repair.
HEADS UP If you're not comfortable with soldering, take it to a small appliance repair shop. This is a straightforward fix for them. Don't attempt it if you're unsure, as you could damage the board.