Sets the starting date time for this gantt chart.
        
        :param      dtime | <QDateTime>

def setDateTimeStart(self, dtime):
        """
        Sets the starting date time for this gantt chart.
        
        :param      dtime | <QDateTime>
        """
        self._dateStart = dtime.date()
        if self.timescale() in (self.Timescale.Minute, self.Timescale.Hour):
            self._timeStart = dtime.time()
        else:
            self._timeStart = QTime(0, 0, 0)