[BUG] Adding offset with nonzero month to DatetimeIndex (GH26258) (#26292)

Author: makbigc

doc/source/whatsnew/v0.25.0.rst

@@ -276,6 +276,7 @@ Datetimelike
 - Improved :class:`Timestamp` type checking in various datetime functions to prevent exceptions when using a subclassed ``datetime`` (:issue:`25851`)
 - Bug in :class:`Series` and :class:`DataFrame` repr where ``np.datetime64('NaT')`` and ``np.timedelta64('NaT')`` with ``dtype=object`` would be represented as ``NaN`` (:issue:`25445`)
 - Bug in :func:`to_datetime` which does not replace the invalid argument with ``NaT`` when error is set to coerce (:issue:`26122`)
+- Bug in adding :class:`DateOffset` with nonzero month to :class:`DatetimeIndex` would raise ``ValueError`` (:issue:`26258`)
 
 Timedelta
 ^^^^^^^^^

pandas/tests/arithmetic/test_datetime64.py

@@ -1435,6 +1435,28 @@ def test_dt64arr_add_sub_offset_ndarray(self, tz_naive_fixture,
         expected = tm.box_expected(expected, box_with_array)
         tm.assert_equal(res, expected)
 
+    @pytest.mark.parametrize("op, offset, exp", [
+        ('__add__', pd.DateOffset(months=3, days=10),
+         DatetimeIndex([Timestamp('2014-04-11'), Timestamp('2015-04-11'),
+                        Timestamp('2016-04-11'), Timestamp('2017-04-11')])),
+        ('__add__', pd.DateOffset(months=3),
+         DatetimeIndex([Timestamp('2014-04-01'), Timestamp('2015-04-01'),
+                        Timestamp('2016-04-01'), Timestamp('2017-04-01')])),
+        ('__sub__', pd.DateOffset(months=3, days=10),
+         DatetimeIndex([Timestamp('2013-09-21'), Timestamp('2014-09-21'),
+                        Timestamp('2015-09-21'), Timestamp('2016-09-21')])),
+        ('__sub__', pd.DateOffset(months=3),
+         DatetimeIndex([Timestamp('2013-10-01'), Timestamp('2014-10-01'),
+                        Timestamp('2015-10-01'), Timestamp('2016-10-01')]))
+
+    ])
+    def test_dti_add_sub_nonzero_mth_offset(self, op, offset, exp):
+        # GH 26258
+        date = date_range(start='01 Jan 2014', end='01 Jan 2017', freq='AS')
+        mth = getattr(date, op)
+        result = mth(offset)
+        tm.assert_equal(result, exp)
+
 
 class TestDatetime64OverflowHandling:
     # TODO: box + de-duplicate

pandas/tseries/offsets.py

@@ -273,7 +273,7 @@ def apply_index(self, i):
                        kwds.get('months', 0)) * self.n)
             if months:
                 shifted = liboffsets.shift_months(i.asi8, months)
-                i = type(i)(shifted, freq=i.freq, dtype=i.dtype)
+                i = type(i)(shifted, dtype=i.dtype)
 
             weeks = (kwds.get('weeks', 0)) * self.n
             if weeks: