re-add timedelta support for polyval (#6599)

Author: headtr1ck

xarray/core/computation.py

@@ -1933,14 +1933,18 @@ def _ensure_numeric(data: T_Xarray) -> T_Xarray:
     from .dataset import Dataset
 
     def to_floatable(x: DataArray) -> DataArray:
-        if x.dtype.kind in "mM":
+        if x.dtype.kind == "M":
+            # datetimes
             return x.copy(
                 data=datetime_to_numeric(
                     x.data,
                     offset=np.datetime64("1970-01-01"),
                     datetime_unit="ns",
                 ),
             )
+        elif x.dtype.kind == "m":
+            # timedeltas
+            return x.astype(float)
         return x
 
     if isinstance(data, Dataset):

xarray/tests/test_computation.py

@@ -2010,6 +2010,14 @@ def test_where_attrs() -> None:
             ),
             id="datetime",
         ),
+        pytest.param(
+            xr.DataArray(
+                np.array([1000, 2000, 3000], dtype="timedelta64[ns]"), dims="x"
+            ),
+            xr.DataArray([0, 1], dims="degree", coords={"degree": [0, 1]}),
+            xr.DataArray([1000.0, 2000.0, 3000.0], dims="x"),
+            id="timedelta",
+        ),
     ],
 )
 def test_polyval(