TST: add tests for using .at() with DateTime indexes (#48286)

* add tests outlined in GH29709

* fix styling error caught by github actions

* made changes based on suggestions from patrick

* fix pep8 mistakes

* remove usage of to_datetime()

* fix error in pre-commit checks also move my new test to the bottom of the class since it was not put in the right place before

Co-authored-by: Neal Muppidi <nealmuppidi@neals-macbook-pro.local>

Author: nealxm

pandas/tests/indexing/test_at.py

@@ -10,6 +10,7 @@
     CategoricalDtype,
     CategoricalIndex,
     DataFrame,
+    DatetimeIndex,
     MultiIndex,
     Series,
     Timestamp,
@@ -109,6 +110,19 @@ def test_at_setitem_multiindex(self):
         )
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize("row", (Timestamp("2019-01-01"), "2019-01-01"))
+    def test_at_datetime_index(self, row):
+        df = DataFrame(
+            data=[[1] * 2], index=DatetimeIndex(data=["2019-01-01", "2019-01-02"])
+        )
+        expected = DataFrame(
+            data=[[0.5, 1], [1.0, 1]],
+            index=DatetimeIndex(data=["2019-01-01", "2019-01-02"]),
+        )
+
+        df.at[row, 0] = 0.5
+        tm.assert_frame_equal(df, expected)
+
 
 class TestAtSetItemWithExpansion:
     def test_at_setitem_expansion_series_dt64tz_value(self, tz_naive_fixture):