QDateEdit and QDateTimeEdit usability improvement

One cannot define and set an invalid date string. QDateEdit, for instance, refuses to accept a string like '-//-'. An example for when this might become necessary is when a database query returns a NULL value for a date. When one uses clear() on the widget, it resets itself to 1/1/00. Of course one could tell the user that 1/1/00 means actually that no date has been entered', but this is an extremely ugly solution.