BUG: Inconsistent output type in Excel for PeriodIndex in Index or MultiIndex

[pforero]

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Reproducible Example

import pandas as pd

period_index = pd.period_range(start="2006-10-06", end="2006-10-07", freq="D")
pi_index = pd.Series(0, index=period_index)
pi_multi = pd.Series(0, index=pd.MultiIndex.from_arrays([period_index, [0, 0]]))

pi_index.to_excel("index_as_date_openpyxl.xlsx", engine="openpyxl", index=True)
pi_multi.to_excel("index_as_str_openpyxl.xlsx", engine="openpyxl", index=True)

pi_index.to_excel("index_as_date_xlsxwriter.xlsx", engine="xlsxwriter",  index=True)
pi_multi.to_excel("index_as_str_xlsxwriter.xlsx", engine="xlsxwriter", index=True)

Issue Description

When exporting a Series/DataFrame to excel when the index contains a PeriodIndex the format of the output depends on the index being an Index or a MultiIndex.

When the PeriodIndex is in an Index, the output format in Excel is a date.
When the PeriodIndex is a level from a MultiIndex, the output format is a string.

Expected Behavior

The expected behaviour is that the format of the period would not be dependent on it being in an Index or in a MultiIndex. Being either a string or a date in excel is both acceptable as long as it is consistent in both outputs.

Installed Versions

INSTALLED VERSIONS

commit : 0691c5c
python : 3.12.4
python-bits : 64
OS : Linux
OS-release : 6.1.100+
Version : #1 SMP PREEMPT_DYNAMIC Sat Oct 5 14:28:44 UTC 2024
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : None
LOCALE : C.UTF-8

pandas : 2.2.3
numpy : 2.0.0
pytz : 2024.1
dateutil : 2.9.0.post0
pip : 24.2
Cython : None
sphinx : None
IPython : 8.25.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : None
lxml.etree : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : 3.1.5
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 16.1.0
pyreadstat : None
pytest : 8.2.2
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : 2.0.31
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : 3.2.0
zstandard : None
tzdata : 2024.1
qtpy : None
pyqt5 : None

[ZKaoChi]

I think this is worth changing, can I try it ?

[rhshadrach]

Thanks for the report. For a non-MultiIndex, we explicitly convert these to timestamps here:

pandas/pandas/io/formats/excel.py

Lines 765 to 766 in d11ed2f

	if isinstance(self.df.index, PeriodIndex):
	index_values = self.df.index.to_timestamp()

We should do something similar in the MultiIndex case. PRs are welcome!