BUG: CustomBusinessMonthBegin(End) sometimes ignores extra offset

[jackzyliu]

• I have checked that this issue has not already been reported (although it is somewhat related to CustomBusinessMonthBegin offset parameter #14869).

• I have confirmed this bug exists on the latest version of pandas. [1.2.4]

• (optional) I have confirmed this bug exists on the master branch of pandas. [1.3.0.dev0+1544.ga43c42c32d.dirty]

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

import pandas as pd
from datetime import timedelta
print(pd.__version__)    # 1.3.0.dev0+1544.ga43c42c32d.dirty

offset_1 = pd.offsets.CustomBusinessMonthBegin(n=1)           # 1 month BMS
offset_1_plus_5d = pd.offsets.CustomBusinessMonthBegin(n=1, offset=timedelta(days=5))   # 1 month BMS + 5 days
offset_2_plus_5d = pd.offsets.CustomBusinessMonthBegin(n=2, offset=timedelta(days=5))   # 2 month BMS + 5 days

test_timestamp = pd.Timestamp('2021-03-01')
print(test_timestamp + offset_1)                # Expected: 2021-04-01, Actual: 2021-04-01 [Correct]
print(test_timestamp + offset_1_plus_5d)  # Expected: 2021-04-06, Actual: 2021-04-01 [Incorrect] 
print(test_timestamp + offset_2_plus_5d)  # Expected: 2021-05-08, Actual: 2021-05-08 [Correct]

Expected Output

I am using 2021-03-01 as the example in the code snippet but I am listing below the expected vs. actual outputs with 2021-04-01 as well.

Date	offset	Expected	Actual	Correct
2021-03-01	offset_1	2021-04-01	2021-04-01	✔️
2021-03-01	offset_1_plus_5d	2021-04-06	2021-04-01	❌
2021-03-01	offset_2_plus_5d	2021-05-08	2021-05-08	✔️
2021-04-01	offset_1	2021-05-03	2021-05-03	✔️
2021-04-01	offset_1_plus_5d	2021-05-08	2021-05-08	✔️
2021-04-01	offset_2_plus_5d	2021-06-06	2021-06-01	❌

Problem description

From reading the implementations, I think the expected behavior of test_timestamp + offset_1_plus_5d for example should be essentially test_timestamp + offset_1 + timedelta(days=5), but we are seeing some inconsistent behaviors.

The issue lies with the .apply logic in _CustomBusinessMonth [link here] , which first rolls with a vanilla MonthBegin or MonthEnd (to variable new) and then uses CustomBusinessDay to roll new to the corresponding business day + extra offset (variable result). However, if new is already on offset, then no further rolling will be applied and no extra offset (e.g. 5 days) will be added either, leading to the inconsistencies we are seeing here. See implementation for rollforward and rollback.

As an example, with CustomBusinessMonthBegin(n=1, offset=timedelta(days=5)), 2021-03-01 first gets rolled to 2021-04-01. Since 2021-04-01 is already a business day, it is not rolled to the next business day and the extra 5-day offset is not applied either.

Since _CustomBusinesMonth is the underlying implementation for both CustomBusinessMonthBegin and CustomBusinessMonthEnd, this issue applies to both classes (although I am only showing CustomBusinessMonthBegin ).

Fix: #41488

Output of pd.show_versions()

INSTALLED VERSIONS

commit : a43c42c
python : 3.8.8.final.0
python-bits : 64
OS : Linux
OS-release : 5.8.0-50-generic
Version : #56~20.04.1-Ubuntu SMP Mon Apr 12 21:46:35 UTC 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.3.0.dev0+1544.ga43c42c32d.dirty
numpy : 1.20.1
pytz : 2021.1
dateutil : 2.8.1
pip : 21.0.1
setuptools : 49.6.0.post20210108
Cython : 0.29.22
pytest : 6.2.2
hypothesis : 6.8.1
sphinx : 3.5.3
blosc : None
feather : None
xlsxwriter : 1.3.7
lxml.etree : 4.6.2
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.3
IPython : 7.21.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : 1.3.2
fsspec : 0.8.7
fastparquet : 0.5.0
gcsfs : 0.7.2
matplotlib : 3.3.4
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.7
pandas_gbq : None
pyarrow : 3.0.0
pyxlsb : None
s3fs : 0.5.2
scipy : 1.6.1
sqlalchemy : 1.4.2
tables : 3.6.1
tabulate : 0.8.9
xarray : 0.17.0
xlrd : 2.0.1
xlwt : 1.3.0
numba : 0.53.0