There is no property you can use in the current version of the RadDatePicker. But using a small trick can help you. Try adding a small TextBlock near to the RadDatePicker and bind it's text to the SelectedDate property of the RadDatePicker using a converter. You can separate this functionality in a UserControl so you can use it in more then one place. Let us know if you need some additional explanation.
All the best,
the Telerik team