The RadDateTimeEditor does not obey MaxDate and MinDate property when the value is changed by keyboard input.

Work around:

public class MyRadDateTimeEditor : RadDateTimeEditor
    {
        public MyRadDateTimeEditor()
        {
            RadDateTimeEditorElement element = this.EditorElement as RadDateTimeEditorElement;
            element.CurrentBehavior.TextBoxElement.ValueChanged += this.OnValueChanged;
        }

        private void OnValueChanged(object sender, System.EventArgs e)
        {
            RadMaskedEditBoxElement maskBox = sender as RadMaskedEditBoxElement;
            RadDateTimeEditorElement element = this.EditorElement as RadDateTimeEditorElement;

            DateTime result;

            if (maskBox.Value != null && DateTime.TryParse(maskBox.Value.ToString(), out result) && DateTime.Compare(result, element.MaxDate) > 0)
            {
                this.Value = element.MaxDate;
                maskBox.Value = element.MaxDate;
                return;
            }
        }
    }