def started(self):
        """Datetime at which the job was started.

        :rtype: ``datetime.datetime``, or ``NoneType``
        :returns: the start time (None until set from the server).
        """
        statistics = self._properties.get("statistics")
        if statistics is not None:
            millis = statistics.get("startTime")
            if millis is not None:
                return _helpers._datetime_from_microseconds(millis * 1000.0)