BUG: CustomBusinessMonthBegin(End) sometimes ignores extra offset (GH41356)

CustomBusinessMonthBegin does not apply extra offset when initially rolled month begin is already a business day.

Author: jackzyliu

pandas/_libs/tslibs/offsets.pyx

@@ -3350,7 +3350,13 @@ cdef class _CustomBusinessMonth(BusinessMixin):
         """
         Define default roll function to be called in apply method.
         """
-        cbday = CustomBusinessDay(n=self.n, normalize=False, **self.kwds)
+        if self.offset:
+            cbday_kwds = self.kwds.copy()
+            cbday_kwds['offset'] = timedelta(0)
+        else:
+            cbday_kwds = self.kwds
+
+        cbday = CustomBusinessDay(n=1, normalize=False, **cbday_kwds)
 
         if self._prefix.endswith("S"):
             # MonthBegin
@@ -3394,6 +3400,9 @@ cdef class _CustomBusinessMonth(BusinessMixin):
 
         new = cur_month_offset_date + n * self.m_offset
         result = self.cbday_roll(new)
+
+        if self.offset:
+            result = result + self.offset
         return result
 
 

pandas/tests/tseries/offsets/test_month.py

@@ -4,6 +4,7 @@
 from datetime import (
     date,
     datetime,
+    timedelta,
 )
 
 import numpy as np
@@ -204,6 +205,56 @@ def test_datetimeindex(self):
             0
         ] == datetime(2012, 1, 31)
 
+    apply_with_extra_offset_cases: _ApplyCases = [
+        (
+            CBMonthEnd(n=1, offset=timedelta(days=5)),
+            {
+                datetime(2021, 3, 1): datetime(2021, 3, 31) + timedelta(days=5),
+                datetime(2021, 4, 17): datetime(2021, 4, 30) + timedelta(days=5),
+            },
+        ),
+        (
+            CBMonthEnd(n=2, offset=timedelta(days=40)),
+            {
+                datetime(2021, 3, 10): datetime(2021, 4, 30) + timedelta(days=40),
+                datetime(2021, 4, 30): datetime(2021, 6, 30) + timedelta(days=40),
+            },
+        ),
+        (
+            CBMonthEnd(n=1, offset=timedelta(days=-5)),
+            {
+                datetime(2021, 3, 1): datetime(2021, 3, 31) - timedelta(days=5),
+                datetime(2021, 4, 11): datetime(2021, 4, 30) - timedelta(days=5),
+            },
+        ),
+        (
+            -2 * CBMonthEnd(n=1, offset=timedelta(days=10)),
+            {
+                datetime(2021, 3, 1): datetime(2021, 1, 29) + timedelta(days=10),
+                datetime(2021, 4, 3): datetime(2021, 2, 26) + timedelta(days=10),
+            },
+        ),
+        (
+            CBMonthEnd(n=0, offset=timedelta(days=1)),
+            {
+                datetime(2021, 3, 2): datetime(2021, 3, 31) + timedelta(days=1),
+                datetime(2021, 4, 1): datetime(2021, 4, 30) + timedelta(days=1),
+            },
+        ),
+        (
+            CBMonthEnd(n=1, holidays=["2021-03-31"], offset=timedelta(days=1)),
+            {
+                datetime(2021, 3, 2): datetime(2021, 3, 30) + timedelta(days=1),
+            },
+        ),
+    ]
+
+    @pytest.mark.parametrize("case", apply_with_extra_offset_cases)
+    def test_apply_with_extra_offset(self, case):
+        offset, cases = case
+        for base, expected in cases.items():
+            assert_offset_equal(offset, base, expected)
+
 
 class TestCustomBusinessMonthBegin(CustomBusinessMonthBase, Base):
     _offset = CBMonthBegin
@@ -341,6 +392,58 @@ def test_datetimeindex(self):
             0
         ] == datetime(2012, 1, 3)
 
+    apply_with_extra_offset_cases: _ApplyCases = [
+        (
+            CBMonthBegin(n=1, offset=timedelta(days=5)),
+            {
+                datetime(2021, 3, 1): datetime(2021, 4, 1) + timedelta(days=5),
+                datetime(2021, 4, 17): datetime(2021, 5, 3) + timedelta(days=5),
+            },
+        ),
+        (
+            CBMonthBegin(n=2, offset=timedelta(days=40)),
+            {
+                datetime(2021, 3, 10): datetime(2021, 5, 3) + timedelta(days=40),
+                datetime(2021, 4, 30): datetime(2021, 6, 1) + timedelta(days=40),
+            },
+        ),
+        (
+            CBMonthBegin(n=1, offset=timedelta(days=-5)),
+            {
+                datetime(2021, 3, 1): datetime(2021, 4, 1) - timedelta(days=5),
+                datetime(2021, 4, 11): datetime(2021, 5, 3) - timedelta(days=5),
+            },
+        ),
+        (
+            -2 * CBMonthBegin(n=1, offset=timedelta(days=10)),
+            {
+                datetime(2021, 3, 1): datetime(2021, 1, 1) + timedelta(days=10),
+                datetime(2021, 4, 3): datetime(2021, 3, 1) + timedelta(days=10),
+            },
+        ),
+        (
+            CBMonthBegin(n=0, offset=timedelta(days=1)),
+            {
+                datetime(2021, 3, 2): datetime(2021, 4, 1) + timedelta(days=1),
+                datetime(2021, 4, 1): datetime(2021, 4, 1) + timedelta(days=1),
+            },
+        ),
+        (
+            CBMonthBegin(
+                n=1, holidays=["2021-04-01", "2021-04-02"], offset=timedelta(days=1)
+            ),
+            {
+                datetime(2021, 3, 2): datetime(2021, 4, 5) + timedelta(days=1),
+            },
+        ),
+    ]
+
+    @pytest.mark.parametrize("case", apply_with_extra_offset_cases)
+    def test_apply_with_extra_offset(self, case):
+        offset, cases = case
+        for base, expected in cases.items():
+            assert_offset_equal(offset, base, expected)
+
 
 class TestSemiMonthEnd(Base):
     _offset = SemiMonthEnd