def epochrange(self, epoch=None, starttime=None, endtime=None):
        """
        Get epoch range.

        Returns a list of the record numbers, representing the
        corresponding starting and ending records within the time
        range from the epoch data. A None is returned if there is no
        data either written or found in the time range.
        """
        return self.varget(variable=epoch, starttime=starttime,
                           endtime=endtime, record_range_only=True)