Returns the date at the given point.
        
        :param      point | <QPoint>

def dateAt( self, point ):
        """
        Returns the date at the given point.
        
        :param      point | <QPoint>
        """
        for date, data in self._dateGrid.items():
            if ( data[1].contains(point) ):
                return QDate.fromJulianDay(date)
        return QDate()