Added comments and updated whatsnew

Author: robdmc

doc/source/whatsnew/v0.17.1.txt

@@ -117,3 +117,5 @@ Bug Fixes
 - Bug in ``to_excel`` with openpyxl 2.2+ and merging (:issue:`11408`)
 
 - Bug in ``DataFrame.to_dict()`` produces a ``np.datetime64`` object instead of ``Timestamp`` when only datetime is present in data (:issue:`11327`)
+
+- Bug in ``pandas.core.groupby`` raises exception when ``func`` in ``df.groupby(...).apply(func)`` doesn't return existing time columns (:issue:`11324`)

pandas/tests/test_groupby.py

@@ -825,7 +825,9 @@ def test_apply_issues(self):
         assert_series_equal(result, expected)
 
     def test_time_field_bug(self):
-        # test a fix for GH issue 11324
+        # Test a fix for the following error related to GH issue 11324
+        # When non-key fields in a group-by dataframe contained time-based fields that
+        # were not returned by the apply function, an exception would be raised.
 
         df = pd.DataFrame({'a': 1,'b': [datetime.now() for nn in range(10)]})