clock and date must be reset every time phone is turned off?

I don't know exactly the construction for clock life . But there must be some power to keep the clock going . Most probably some kind of condensor is doing this . Even when the main battery is removed , clock stays alive a while .
This condensor must be dead I suppose .
Instead of closing the complete Phone , you could set it to Offline .
Press powerbutton shortly > Select : Offline .

😊 Regards jApi NL