BUG: guess_datetime_format cannot infer iso 8601 format

[Thomath]

Pandas version checks

• I have checked that this issue has not already been reported.

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

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd 

# UserWarning: Could not infer format, so each element will be parsed individually, falling back to `dateutil`. To ensure parsing is consistent and as-expected, please specify a format.
pd.to_datetime(
    pd.Series(['2025-05-05 20:25:22+00:00', '2025-05-05 12:04:52+00:00'])
)

# no warning
pd.to_datetime(
    pd.Series(['2025-05-05 20:25:22+00:00'])
)

# No warning
pd.to_datetime(
    pd.Series(['2025-05-05 12:03:08+00:00', '2025-05-05 12:04:52+00:00']),
)

Issue Description

When running pd.to_datetime(pd.Series(['2025-05-05 20:25:22+00:00', '2025-05-05 12:04:52+00:00'])) the following warning is risen:

UserWarning: Could not infer format, so each element will be parsed individually, falling back to dateutil. To ensure parsing is consistent and as-expected, please specify a format.

This is because guess_datetime_format cannot infer a format for the first given timestamp '2025-05-05 20:25:22+00:00'.

Expected Behavior

No warning is risen.

Installed Versions

INSTALLED VERSIONS

commit : 0691c5c
python : 3.12.10
python-bits : 64
OS : Linux
OS-release : 6.8.0-58-generic
Version : #60~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Fri Mar 28 16:09:21 UTC 2
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.2.3
numpy : 1.26.4
pytz : 2025.2
dateutil : 2.9.0.post0
pip : 25.0.1
Cython : None
sphinx : None
IPython : None
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 : 3.1.6
lxml.etree : None
matplotlib : 3.10.1
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : 2.9.10
pymysql : None
pyarrow : None
pyreadstat : None
pytest : 8.3.5
python-calamine : None
pyxlsb : None
s3fs : None
scipy : 1.15.2
sqlalchemy : 2.0.40
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2025.2
qtpy : None
pyqt5 : None

[rhshadrach]

Thanks for the report. I can reproduce on 2.2.x, but this warning does not appear on main. This will be fixed in pandas 3.0. Closing.