BUG: BusinessDay addition with non-nano (#55608)

* BUG: BusinessDay addition with non-nano

* GH ref

* fix nanos case

doc/source/whatsnew/v2.2.0.rst

@@ -324,6 +324,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.union` returning object dtype for tz-aware indexes with the same timezone but different units (:issue:`55238`)
 - Bug in :meth:`Tick.delta` with very large ticks raising ``OverflowError`` instead of ``OutOfBoundsTimedelta`` (:issue:`55503`)
 - Bug in adding or subtracting a :class:`Week` offset to a ``datetime64`` :class:`Series`, :class:`Index`, or :class:`DataFrame` column with non-nanosecond resolution returning incorrect results (:issue:`55583`)
+- Bug in addition or subtraction of :class:`BusinessDay` offset with ``offset`` attribute to non-nanosecond :class:`Index`, :class:`Series`, or :class:`DataFrame` column giving incorrect results (:issue:`55608`)
 - Bug in addition or subtraction of :class:`DateOffset` objects with microsecond components to ``datetime64`` :class:`Index`, :class:`Series`, or :class:`DataFrame` columns with non-nanosecond resolution (:issue:`55595`)
 - Bug in addition or subtraction of very large :class:`Tick` objects with :class:`Timestamp` or :class:`Timedelta` objects raising ``OverflowError`` instead of ``OutOfBoundsTimedelta`` (:issue:`55503`)
 -

pandas/_libs/tslibs/offsets.pyx

@@ -1850,7 +1850,6 @@ cdef class BusinessDay(BusinessMixin):
         res = self._shift_bdays(i8other, reso=reso)
         if self.offset:
             res = res.view(dtarr.dtype) + Timedelta(self.offset)
-            res = res.view("i8")
         return res
 
     def is_on_offset(self, dt: datetime) -> bool:

pandas/tests/tseries/offsets/test_business_hour.py

@@ -984,9 +984,17 @@ def test_short_datetimeindex_creation(self):
         expected4 = DatetimeIndex(["2014-07-01 10:00"], freq="bh")
         tm.assert_index_equal(idx4, expected4)
 
-    def test_bday_ignores_timedeltas(self):
-        idx = date_range("2010/02/01", "2010/02/10", freq="12h")
-        t1 = idx + BDay(offset=Timedelta(3, unit="h"))
+    @pytest.mark.parametrize("td_unit", ["s", "ms", "us", "ns"])
+    @pytest.mark.parametrize("unit", ["s", "ms", "us", "ns"])
+    def test_bday_ignores_timedeltas(self, unit, td_unit):
+        # GH#55608
+        idx = date_range("2010/02/01", "2010/02/10", freq="12h", unit=unit)
+        td = Timedelta(3, unit="h").as_unit(td_unit)
+        off = BDay(offset=td)
+        t1 = idx + off
+
+        exp_reso = max(td._creso, idx._data._creso)
+        exp_unit = {7: "s", 8: "ms", 9: "us", 10: "ns"}[exp_reso]
 
         expected = DatetimeIndex(
             [
@@ -1011,9 +1019,22 @@ def test_bday_ignores_timedeltas(self):
                 "2010-02-11 03:00:00",
             ],
             freq=None,
-        )
+        ).as_unit(exp_unit)
         tm.assert_index_equal(t1, expected)
 
+        # TODO(GH#55564): as_unit will be unnecessary
+        pointwise = DatetimeIndex([x + off for x in idx]).as_unit(exp_unit)
+        tm.assert_index_equal(pointwise, expected)
+
+    def test_add_bday_offset_nanos(self):
+        # GH#55608
+        idx = date_range("2010/02/01", "2010/02/10", freq="12h", unit="ns")
+        off = BDay(offset=Timedelta(3, unit="ns"))
+
+        result = idx + off
+        expected = DatetimeIndex([x + off for x in idx])
+        tm.assert_index_equal(result, expected)
+
 
 class TestOpeningTimes:
     # opening time should be affected by sign of n, not by n's value and end