The DateTimePicker control is designed to have free text box where you can type arbitrary strings and the control tries to parse your input. We are not currently planning to add masked behavior to it as this will limit typing strings like "Monday" or "tomorrow". Though we will consider your suggestion.
What I could suggest you is to use a MaskedTextBox, a DropDownButton and a Calendar in the DropDownButton - this is similar to what we do in the DateTimePicker control.
All the best,
the Telerik team
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