We consider setting the Value of the DateTimePicker to a previous valid Value as more of application logic that should be handled according to the specific scenario.
You could implement validation in the ViewModel with the IDataErrorInfo interface as demonstrated in the DateTimlePicker Data Validation example inside WPF Demos
or you could just check the SelectedValue
and preserve the last valid value in some variable that can be used in case SelectedValue is null.
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