'dT(X) +/- dT2(Y) -- Add/substract JDs for dT(X) and dT(Y).X
I.e. this will succeed if dT(Y) can be converted to scale X.
Returns delta time in scale X'

def test_scales_for_delta_minus_delta(self):

    dt_tai = self.dt['tai']
    dt_tt = self.dt['tt']
    dt0 = (dt_tai - dt_tt)
    assert (dt0.scale == 'tai')
    assert allclose_sec(dt0.sec, 0.0)
    dt_tcg = self.dt['tcg']
    dt1 = (dt_tai - dt_tcg)
    assert (dt1.scale == 'tai')
    assert (not allclose_sec(dt1.sec, 0.0))
    t_tai_tcg = self.t['tai'].tcg
    dt_tai_tcg = (t_tai_tcg - t_tai_tcg[0])
    dt2 = (dt_tai - dt_tai_tcg)
    assert (dt2.scale == 'tai')
    assert allclose_sec(dt2.sec, 0.0)
    dt3 = (dt_tai_tcg - dt_tai)
    assert (dt3.scale == 'tcg')
    assert allclose_sec(dt3.sec, 0.0)
    for scale in ('tdb', 'tcb', 'ut1'):
        with pytest.raises(TypeError):
            (dt_tai - self.dt[scale])
    dt_tcb = self.dt['tcb']
    dt_tdb = self.dt['tdb']
    dt4 = (dt_tcb - dt_tdb)
    assert (dt4.scale == 'tcb')
    assert (not allclose_sec(dt1.sec, 0.0))
    t_tcb_tdb = self.t['tcb'].tdb
    dt_tcb_tdb = (t_tcb_tdb - t_tcb_tdb[0])
    dt5 = (dt_tcb - dt_tcb_tdb)
    assert (dt5.scale == 'tcb')
    assert allclose_sec(dt5.sec, 0.0)
    for scale in ('utc', 'tai', 'tt', 'tcg', 'ut1'):
        with pytest.raises(TypeError):
            (dt_tcb - self.dt[scale])
    dt_ut1 = self.dt['ut1']
    dt5 = (dt_ut1 - dt_ut1[(-1)])
    assert (dt5.scale == 'ut1')
    assert (dt5[(-1)].sec == 0.0)
    for scale in ('utc', 'tai', 'tt', 'tcg', 'tcb', 'tdb'):
        with pytest.raises(TypeError):
            (dt_ut1 - self.dt[scale])
