System Clock and Calender do not work

I’ve had this DSI for a number of years and for the last few my system clock hasn’t worked. I havn’t been playing it much so it hasn’t been an issue until now. Any time based events in games do not work, my time will not move allowing the days to change and day night cycles don’t happen, and when I open the alarm feature it doesn’t count down at all. No modifications have been made to the device, Any Help?