I'm not sure what stereo you might have in you van.
I have a 97 t&c
I have the infinity stereo :headbange
On the front panel of the stereo, you should find two small holes. One is marked hours and the other is marked minutes. Use a tooth pick (or something small and not sharp) and push in the small button that is in the matching hole.
I have a owners manuel, just let me know what stereo you have, and I will look it up.