The DateFormatString of the RadDateInput/RadDatePicker should have all the day, month and year parts. You can use the DisplayDateFormatString property to specify how date would look when the control is out of focus. If this is not applicable for your case, you would better use regular textbox control with RadCalendar for picking dates.
Additionally, you can refer to the below code library projects for further alternatives:
All the best,
the Telerik team
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