My point and shoot camera used to keep the date when turned off. Now, every time I turn it on, I have to reset the date and time. What could cause this?
The internal clock in a camera requires power to keep running. In some cameras, the clock runs off a separate battery from the main one. This secondary clock battery (often a small button cell) may have died and needs to be replaced.
For an example of this type of battery replacement, see the ifixit.com guide on replacing the clock battery in a Canon Powershot A590.