time date setting

Hi, I know I have a old Aptus II but every time I put it off and on, I need to set the date and time. I thougt the back had an accu on it, so I put it on a powersupply for 24 houres: no effect.  It is not that that is a big issue but I want to know if damage will evolve during the years bacause a malfunction accu is -for example - leaking accid. Best regards Henk.