What causes the stereo clock to re-set itself to 1200 every time you start the car?

There is a wire to the stereo that has power all the time ( straight to battery ) so that it retains its memory. This wire is disconnected, broken, or faulted somewhere. If it is an aftermarket stereo it might never have been connected. In the stereo wiring harness there should be one wire that has power with the ignition off, that is the one it will connect to for memory. If there is not one start checking fuses and wire continuity.