Q: I left my Surface device unplugged for a period of time; when I charged it and turned it back on, it had lost the date and time. Why did this happen?
A: The Surface devices don't have the "button" battery that's common in many computers to keep the real-time clock current even when the main power support/battery of the computer is empty. This means if the Surface battery is completely drained, it's no longer possible to power the real-time clock. Therefore, when the computer is charged again and is restarted, it will have lost the date and time and will require that information to be reset, unless it's configured to use an external time source.