Before non-removable battery became mainstream, most phones had a small internal battery to keep the internal clock ticking when the battery is being swapped. This is not unlike the CMOS battery found on PC motherboards.<p>iPhones does not need this backup power since the battery is always attached. Hence in case the battery becomes completely drained the internal clock will reset. I can&#x27;t recall what the default time iOS resorts to after a total power loss but IIRC it was a date well into the 2000s.