Rule-Edit-Input will allow you to modify what has been typed and insert the slashes. You can add this to your property definition. I'm not sure how this would mesh with the dateTime control itself -- it might be incompatible.
Another possibility is to add an "up key" event to run a script after every key stroke; you can do this in the Action List on the property. Your script would analyze the content so far typed, and add the slashes as necessary. This might be closer to what you are looking for.