Thank you for contacting Telerik support.
RadDateTimePicker is a strongly validated control - when the user types the date in, the value of the control is a valid date. This is a desired behavior and cannot be modified.
I would like to suggest using RadMaskedEditBox editor with MaskType = Standard and Mask = "##/##/####", instead of RadDateTimePicker editor. The disadvantage of this scenario is that you should create validation logic for user input (for example 31/06/2012 which is incorrect).
I hope this helps.
All the best,
the Telerik team