Add whatsnew, use np.atleast_2d, add comment

Author: watercrossing

doc/source/whatsnew/v0.21.1.txt

@@ -60,6 +60,7 @@ Bug Fixes
 - Bug in :class:`TimedeltaIndex` subtraction could incorrectly overflow when ``NaT`` is present (:issue:`17791`)
 - Bug in :class:`DatetimeIndex` subtracting datetimelike from DatetimeIndex could fail to overflow (:issue:`18020`)
 - Bug in ``pd.Series.rolling.skew()`` and ``rolling.kurt()`` with all equal values has floating issue (:issue:`18044`)
+- Bug in ``pd.DataFrameGroupBy.count()`` when counting over a datetimelike column (:issue:`13393`)
 
 Conversion
 ^^^^^^^^^^

pandas/core/groupby.py

@@ -4365,9 +4365,7 @@ def count(self):
         ids, _, ngroups = self.grouper.group_info
         mask = ids != -1
 
-        makeNDarray = lambda vals: vals[None, :] if vals.ndim == 1 else vals
-
-        val = ((mask & ~isna(makeNDarray(blk.get_values())))
+        val = ((mask & ~isna(np.atleast_2d(blk.get_values())))
                for blk in data.blocks)
         loc = (blk.mgr_locs for blk in data.blocks)
 

pandas/tests/groupby/test_counting.py

@@ -197,6 +197,7 @@ def test_ngroup_respects_groupby_order(self):
                                 g.cumcount())
 
     def test_count_with_datetime(self):
+        # test for #13393
         df = DataFrame({'x': ['a', 'a', 'b'],
                         'y': [Timestamp('2016-05-07 20:09:25+00:00'),
                               Timestamp('2016-05-07 20:09:29+00:00'),