Add a non-ns test for polyval, polyfit

xarray/tests/test_computation.py

@@ -2343,6 +2343,20 @@ def test_where_attrs() -> None:
             ),
             id="datetime",
         ),
+        pytest.param(
+            # Force a non-ns unit for the coordinate, make sure we convert to `ns`
+            # for backwards compatibility at the moment. This can be relaxed in the future.
+            xr.DataArray(
+                pd.date_range("1970-01-01", freq="s", periods=3, unit="s"), dims="x"
+            ),
+            xr.DataArray([0, 1], dims="degree", coords={"degree": [0, 1]}),
+            xr.DataArray(
+                [0, 1e9, 2e9],
+                dims="x",
+                coords={"x": pd.date_range("1970-01-01", freq="s", periods=3)},
+            ),
+            id="datetime-non-ns",
+        ),
         pytest.param(
             xr.DataArray(
                 np.array([1000, 2000, 3000], dtype="timedelta64[ns]"), dims="x"
@@ -2457,6 +2471,14 @@ def test_polyval_degree_dim_checks() -> None:
             xr.DataArray(pd.date_range("1970-01-01", freq="ns", periods=3), dims="x"),
             id="datetime",
         ),
+        # Force a non-ns unit for the coordinate, make sure we convert to `ns` in both polyfit & polval
+        # for backwards compatibility at the moment. This can be relaxed in the future.
+        pytest.param(
+            xr.DataArray(
+                pd.date_range("1970-01-01", freq="s", unit="s", periods=3), dims="x"
+            ),
+            id="datetime-non-ns",
+        ),
         pytest.param(
             xr.DataArray(np.array([0, 1, 2], dtype="timedelta64[ns]"), dims="x"),
             id="timedelta",