Hi @serene1 ,
Given that the date and time settings are only held for 10 hours if the main battery is removed, it is more than likely that a capacitor is being used to maintain the settings rather than a rechargeable battery.
Without a schematic diagram or a service manual it will be difficult to know where it is.
I have looked at teardown videos of the camera on YouTube but cannot see anything obvious. There's certainly nothing that looks like a battery, directly mounted on the motherboard like they have in other cameras but it may be somewhere else in the camera.