To reproduce:
            radDateTimePicker1.MinDate = new DateTime(2013, 9, 1);
            radDateTimePicker1.MaxDate = new DateTime(2013, 9, 15);
            radDateTimePicker1.NullDate = new DateTime(2013, 9, 1);
            radDateTimePicker1.NullText = "text";

Start the app and set the year to "1111", then click some button to cause the date time picker to lose focus. At this point, print the control Text and Value. While the text is correct, the value shows with year "1111" and is not validated by the MinDate 2013/9/1

Workaround - work with null and NullableValue instead of NullDate and a specific date, or use the following class

   class MyRadDateTimePicker : RadDateTimePicker
        {
            public override DateTime Value
            {
                get
                {
                    DateTime? date = this.DateTimePickerElement.Value;
                    if (date.HasValue)
                    {
                        if (date < this.MinDate)
                        {
                            return this.MinDate;
                        }

                        if (date > this.MaxDate)
                        {
                            return this.MaxDate;
                        }

                        return date.Value;
                    }

                    return this.MinDate;
                }
                set
                {
                    base.Value = value;
                }
            }