docs: 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)