well all portable devices have a small battery soldered into the main board to keep your devices time & date alive 24/7....if the battery is dead then your device will depends on your main battery to make sure the time & date is not reset every time you turn of your devices...

this small battery can be replaced at any phone repair shop...you can replaced your self if you know how to solder it...

FYI...if your devices time & date got reset then that is the date that your devices have been made/manufacture....