BUG: Behavior change in pandas 3.x with Series.resample and closed="right", how="right" #62200
Description
Pandas version checks
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I have checked that this issue has not already been reported.
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I have confirmed this bug exists on the latest version of pandas.
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I have confirmed this bug exists on the main branch of pandas.
Reproducible Example
import pandas as pd
import pandas.testing
index = pd.date_range("1-1-2000", "2-15-2000", freq="h").union(pd.date_range("4-15-2000", "5-15-2000", freq="h"))
s = pd.Series(range(len(index)), index=index)
left = s.resample("D", label="right", closed="right").count()
right = s.resample("24h", label="right", closed="right").count()
pandas.testing.assert_series_equal(left, right)Issue Description
That program passes without error with pandas==2.3.2, but fails with 3.0.0.dev0+2339.g1af0a95e94. The failure is
AssertionError: Series are different
Series length are different
[left]: 137, DatetimeIndex(['2000-01-01', '2000-01-02', '2000-01-03', '2000-01-04',
'2000-01-05', '2000-01-06', '2000-01-07', '2000-01-08',
'2000-01-09', '2000-01-10',
...
'2000-05-07', '2000-05-08', '2000-05-09', '2000-05-10',
'2000-05-11', '2000-05-12', '2000-05-13', '2000-05-14',
'2000-05-15', '2000-05-16'],
dtype='datetime64[ns]', length=137, freq='D')
[right]: 136, DatetimeIndex(['2000-01-01', '2000-01-02', '2000-01-03', '2000-01-04',
'2000-01-05', '2000-01-06', '2000-01-07', '2000-01-08',
'2000-01-09', '2000-01-10',
...
'2000-05-06', '2000-05-07', '2000-05-08', '2000-05-09',
'2000-05-10', '2000-05-11', '2000-05-12', '2000-05-13',
'2000-05-14', '2000-05-15'],
dtype='datetime64[ns]', length=136, freq='24h')
Expected Behavior
Uncertain. I think this is related to #35248 and the fix from #61985. I'm not 100% sure whether the old behavior was buggy and this was a deliberate change. But #35248 mentions "24h" and "1D" behaving differently, so perhaps this is a regresssion.
Installed Versions
INSTALLED VERSIONS
------------------
commit : 1af0a95e946bfbba814bc5b395cf65047fcac5df
python : 3.12.8
python-bits : 64
OS : Darwin
OS-release : 24.6.0
Version : Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:40 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6041
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8
pandas : 3.0.0.dev0+2339.g1af0a95e94
numpy : 2.4.0.dev0+git20250808.622f874
dateutil : 2.9.0.post0
pip : None
Cython : None
sphinx : None
IPython : 9.4.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
bottleneck : None
fastparquet : None
fsspec : 2025.3.2
html5lib : None
hypothesis : 6.136.1
gcsfs : None
jinja2 : 3.1.6
lxml.etree : None
matplotlib : 3.10.3
numba : None
numexpr : None
odfpy : None
openpyxl : None
psycopg2 : None
pymysql : None
pyarrow : 21.0.0
pyiceberg : None
pyreadstat : None
pytest : 8.4.1
python-calamine : None
pytz : 2025.2
pyxlsb : None
s3fs : 2025.7.0
scipy : None
sqlalchemy : 2.0.41
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
qtpy : None
pyqt5 : None