If the user is entering the date manually, then a DTPicker is probably not the right control to use. Maybe a standard TextBox will work for you.

On the other hand, if you must use a DTPicker, you'll need to 'customize' it to provide the behavior you want. I've used a great NullableDateTimePicker by Claudio Grazioli (www.grazioli.ch) that might help you create your own version that allows what you need.

HTH