Given a recarr, slice out the given artic.date.DateRange if a
        datetime64 index exists

def _daterange(self, recarr, date_range):
        """ Given a recarr, slice out the given artic.date.DateRange if a
        datetime64 index exists """
        idx = self._datetime64_index(recarr)
        if idx and len(recarr):
            dts = recarr[idx]
            mask = Series(np.zeros(len(dts)), index=dts)
            start, end = _start_end(date_range, dts)
            mask[start:end] = 1.0
            return recarr[mask.values.astype(bool)]
        return recarr