def modified(self):
        """Union[datetime.datetime, None]: Datetime at which the dataset was
        last modified (:data:`None` until set from the server).
        """
        modified_time = self._properties.get("lastModifiedTime")
        if modified_time is not None:
            # modified_time will be in milliseconds.
            return google.cloud._helpers._datetime_from_microseconds(
                1000.0 * float(modified_time)
            )