def test_get_clean_interp_index_dt(cf_da, calendar, freq) -> None:
    """In the gregorian case, the index should be proportional to normal datetimes."""
    g = cf_da(calendar, freq=freq)
    g["stime"] = xr.Variable(
        data=g.time.to_index().to_datetimeindex(time_unit="ns"), dims=("time",)
    )

    gi = get_clean_interp_index(g, "time")
    si = get_clean_interp_index(g, "time", use_coordinate="stime")
    np.testing.assert_array_equal(gi, si)
In the gregorian case, the index should be proportional to normal datetimes.
xarray/tests/test_missing.py