Bugfix/groupby datetime issue

doc/source/whatsnew/v1.0.0.rst

@@ -181,6 +181,7 @@ Datetimelike
 - Bug in :class:`Series` and :class:`DataFrame` with integer dtype failing to raise ``TypeError`` when adding or subtracting a ``np.datetime64`` object (:issue:`28080`)
 - Bug in :class:`Week` with ``weekday`` incorrectly raising ``AttributeError`` instead of ``TypeError`` when adding or subtracting an invalid type (:issue:`28530`)
 - Bug in :class:`DataFrame` arithmetic operations when operating with a :class:`Series` with dtype `'timedelta64[ns]'` (:issue:`28049`)
+- Bug in :func:`pandas.core.groupby.generic._recast_datetimelike_result` integer indexes for columns referenced by name (:issue:`28247`)
 -
 
 Timedelta

pandas/core/groupby/generic.py

@@ -1904,9 +1904,8 @@ def _recast_datetimelike_result(result: DataFrame) -> DataFrame:
     """
     result = result.copy()
 
-    obj_cols = [
-        idx for idx in range(len(result.columns)) if is_object_dtype(result.dtypes[idx])
-    ]
+    obj_cols = [idx for idx in range(len(result.columns))
+                if is_object_dtype(result.dtypes.iloc[idx])]
 
     # See GH#26285
     for n in obj_cols:

pandas/tests/groupby/test_apply.py

@@ -657,3 +657,14 @@ def test_apply_with_mixed_types():
 
     result = g.apply(lambda x: x / x.sum())
     tm.assert_frame_equal(result, expected)
+
+
+def test_apply_datetime_issue():
+    # GH-28247
+
+    df = pd.DataFrame({'a': ['foo'], 'b': [datetime.today()]})
+    result = df.groupby('a').apply(lambda x: pd.Series(['spam'], index=[42]))
+
+    expected = pd.DataFrame(['spam'], Index(['foo'], dtype='object', name='a'),
+                            columns=[42])
+    tm.assert_frame_equal(result, expected)