BUG: groupby.transform retains timezone information

doc/source/whatsnew/v0.24.2.rst

@@ -78,7 +78,7 @@ Bug Fixes
 
 **Reshaping**
 
--
+- Bug in :meth:`pandas.core.groupby.GroupBy.transform` where applying a function to a timezone aware column would return a timezone naive result (:issue:`24198`)
 - Bug in :func:`DataFrame.join` when joining on a timezone aware :class:`DatetimeIndex` (:issue:`23931`)
 -
 

pandas/core/groupby/generic.py

@@ -964,7 +964,7 @@ def _transform_fast(self, func, func_nm):
 
         ids, _, ngroup = self.grouper.group_info
         cast = self._transform_should_cast(func_nm)
-        out = algorithms.take_1d(func().values, ids)
+        out = algorithms.take_1d(func()._values, ids)
         if cast:
             out = self._try_cast(out, self.obj)
         return Series(out, index=self.obj.index, name=self.obj.name)

pandas/tests/groupby/test_transform.py

@@ -834,3 +834,14 @@ def demean_rename(x):
     tm.assert_frame_equal(result, expected)
     result_single = df.groupby('group').value.transform(demean_rename)
     tm.assert_series_equal(result_single, expected['value'])
+
+
+@pytest.mark.parametrize('func', [min, max, np.min, np.max])
+def test_groupby_transform_timezone_column(func):
+    # GH 24198
+    ts = pd.to_datetime('now', utc=True).tz_convert('Asia/Singapore')
+    result = pd.DataFrame({'end_time': [ts], 'id': [1]})
+    result['max_end_time'] = result.groupby('id').end_time.transform(func)
+    expected = pd.DataFrame([[ts, 1, ts]], columns=['end_time', 'id',
+                                                    'max_end_time'])
+    tm.assert_frame_equal(result, expected)