'T(X) +/- dT(None) or T(X) +/- Quantity(time-like)
This is always allowed and just adds JDs, i.e., the scale of
the TimeDelta or time-like Quantity will be taken to be X.
The one exception is again for X=UTC, where TAI is assumed instead,
so that a day is always defined as 86400 seconds.'

@pytest.mark.parametrize(('scale', 'op'), list(itertools.product(TIME_SCALES, (operator.add, operator.sub))))
def test_scales_for_delta_scale_is_none(self, scale, op):

    dt_none = TimeDelta([0.0, 1.0, (-1.0), 1000.0], format='sec')
    assert (dt_none.scale is None)
    q_time = dt_none.to('s')
    dt = self.dt[scale]
    dt1 = op(dt, dt_none)
    assert (dt1.scale == dt.scale)
    assert allclose_jd(dt1.jd, op(dt.jd, dt_none.jd))
    dt2 = op(dt_none, dt)
    assert (dt2.scale == dt.scale)
    assert allclose_jd(dt2.jd, op(dt_none.jd, dt.jd))
    dt3 = op(q_time, dt)
    assert (dt3.scale == dt.scale)
    assert allclose_jd(dt3.jd, dt2.jd)
    t = self.t[scale]
    t1 = op(t, dt_none)
    assert (t1.scale == t.scale)
    assert allclose_jd(t1.jd, op(t.jd, dt_none.jd))
    if (op is operator.add):
        t2 = op(dt_none, t)
        assert (t2.scale == t.scale)
        assert allclose_jd(t2.jd, t1.jd)
    t3 = op(t, q_time)
    assert (t3.scale == t.scale)
    assert allclose_jd(t3.jd, t1.jd)
