By design the whenever the input cannot be parsed to valid value, the SelectedDate property are set to null. However whenever the SelectedDate is bound to non nullable DateTime - it is not possible the property to be set to null that is why it is not changed.
What you can do in this scenario would be to hook to the ParseDateTimeValue
event and check in the arguments whether the parsing is successful. If not you could simply revert the selected value to the previous one:
sender, Telerik.Windows.Controls.ParseDateTimeEventArgs args)
args.Result = args.PreviousValue;
Hope this helps.
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