Fix for date offset strings with resample loffset (#8422)

* change back to using to_offset in first_items

* add date offset to loffset test

* add to release notes

doc/whats-new.rst

@@ -50,6 +50,9 @@ Bug fixes
   ``"right"`` to xarray's implementation of resample for data indexed by a
   :py:class:`CFTimeIndex` (:pull:`8393`).
   By `Spencer Clark <https://github.com/spencerkclark>`_.
+- Fix to once again support date offset strings as input to the loffset
+  parameter of resample and test this functionality (:pull:`8422`, :issue:`8399`).
+  By `Katelyn FitzGerald <https://github.com/kafitzgerald>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/resample_cftime.py

@@ -151,7 +151,10 @@ def first_items(self, index: CFTimeIndex):
                     f"Got {self.loffset}."
                 )
 
-            labels = labels + pd.to_timedelta(self.loffset)
+            if isinstance(self.loffset, datetime.timedelta):
+                labels = labels + self.loffset
+            else:
+                labels = labels + to_offset(self.loffset)
 
         # check binner fits data
         if index[0] < datetime_bins[0]:

xarray/tests/test_cftimeindex_resample.py

@@ -260,7 +260,7 @@ def test_timedelta_offset() -> None:
     xr.testing.assert_identical(timedelta_result, string_result)
 
 
-@pytest.mark.parametrize("loffset", ["12H", datetime.timedelta(hours=-12)])
+@pytest.mark.parametrize("loffset", ["MS", "12H", datetime.timedelta(hours=-12)])
 def test_resample_loffset_cftimeindex(loffset) -> None:
     datetimeindex = pd.date_range("2000-01-01", freq="6H", periods=10)
     da_datetimeindex = xr.DataArray(np.arange(10), [("time", datetimeindex)])