core: try coerce result back to DatetimeBlock

doc/source/whatsnew/v0.24.0.txt

@@ -436,6 +436,7 @@ Datetimelike
 
 - Fixed bug where two :class:`DateOffset` objects with different ``normalize`` attributes could evaluate as equal (:issue:`21404`)
 - Fixed bug where :meth:`Timestamp.resolution` incorrectly returned 1-microsecond ``timedelta`` instead of 1-nanosecond :class:`Timedelta` (:issue:`21336`,:issue:`21365`)
+- Fixed bug where :class:`DataFrame` with ``dtype='datetime64[ns]'`` operating with :class:`DateOffset` could cast to ``dtype='object'`` (:issue:`21610`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/internals/__init__.py

@@ -2737,7 +2737,7 @@ def _try_coerce_args(self, values, other):
 
     def _try_coerce_result(self, result):
         """ reverse of try_coerce_args """
-        if isinstance(result, np.ndarray):
+        if isinstance(result, (np.ndarray, Block)):
             if result.dtype.kind in ['i', 'f', 'O']:
                 try:
                     result = result.astype('M8[ns]')
@@ -2785,6 +2785,12 @@ def set(self, locs, values, check=False):
 
         self.values[locs] = values
 
+    def eval(self, try_cast=False, *args, **kwargs):
+        blocks = super().eval(try_cast=try_cast, *args, **kwargs)
+        if try_cast:
+            blocks = [self._try_coerce_result(block) for block in blocks]
+        return blocks
+
 
 class DatetimeTZBlock(NonConsolidatableMixIn, DatetimeBlock):
     """ implement a datetime64 block with a tz attribute """
@@ -2920,6 +2926,8 @@ def _try_coerce_result(self, result):
         if isinstance(result, np.ndarray):
             if result.dtype.kind in ['i', 'f', 'O']:
                 result = result.astype('M8[ns]')
+        elif isinstance(result, Block):
+            result = self.make_block_same_class(result.values.flat)
         elif isinstance(result, (np.integer, np.float, np.datetime64)):
             result = tslibs.Timestamp(result, tz=self.values.tz)
         if isinstance(result, np.ndarray):

pandas/tests/frame/test_arithmetic.py

@@ -211,6 +211,16 @@ def test_df_sub_datetime64_not_ns(self):
                                  pd.Timedelta(days=2)])
         tm.assert_frame_equal(res, expected)
 
+    def test_timestamp_df_add_dateoffset(self):
+        expected = pd.DataFrame([pd.Timestamp('2019')])
+        result = pd.DataFrame([pd.Timestamp('2018')]) + pd.DateOffset(years=1)
+        tm.assert_frame_equal(expected, result)
+
+        expected = pd.DataFrame([pd.Timestamp('2019', tz='Asia/Shanghai')])
+        result = (pd.DataFrame([pd.Timestamp('2018', tz='Asia/Shanghai')])
+                  + pd.DateOffset(years=1))
+        tm.assert_frame_equal(expected, result)
+
     @pytest.mark.parametrize('data', [
         [1, 2, 3],
         [1.1, 2.2, 3.3],