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

def dateTimeAt( self, point ):
        """
        Returns the date time at the inputed point.
        
        :param      point | <QPoint>
        """
        for dtime, data in self._dateTimeGrid.items():
            if ( data[1].contains(point) ):
                return QDateTime.fromTime_t(dtime)
        return QDateTime()