You are trying to dictate the generic string representation of a java.util.Date object in your dataset. No can do. (Though you might place java.sql.Timestamp objects in their place--subclasses of java.util.Date that have a generic string representation closer to what you want.)
Don't convert to string! (In the dataset. Convert to string at the point of display only.)
String conversions are the source of endless grief and frustration.