Problems with older DS

I just picked up an older ds that when it starts up, asks you all the info questions (time, date, etc...)but when it tells you to restart the system by hitting A, it goes off and when you turn it back on, you have to reenter all the info again. If you put a game in it, it plays fine. Any ideas?

The Older DS's had internal batteries i them (similar to PC motherboard ones) for time keeping etc.

The battery has probably drained.