BUG: UserWarning about lacking infer_datetime_format with pd.to_datetime

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Reproducible Example

df['date'] = pd.to_datetime(df['date'], infer_datetime_format=True, utc=True, errors='ignore')

Issue Description

When I run this code, I get the following warning:

“UserWarning: Parsing ‘15/09/1979’ in DD/MM/YYYY format. Provide format or specify infer_datetime_format=True for consistent parsing.”

Which is strange, because I already have set infer_datetime_format=True.

Expected Behavior

You would expect no warning at all, since “infer_datetime_format=True” is already provided as an argument.

Installed Versions

pymysql : None html5lib : None lxml.etree : 4.8.0 xlsxwriter : None feather : None blosc : None sphinx : None hypothesis : None pytest : 6.2.5 Cython : None setuptools : 58.1.0 pip : 22.0.3 dateutil : 2.8.2 pytz : 2021.3 numpy : 1.22.2 pandas : 1.4.1

[MarcoGorelli]

Reckon this would be clearer?

>>> import pandas as pd
>>> pd.to_datetime(['01/01/2000','31/05/2000','31/05/2001', '01/02/2000'], infer_datetime_format=True)
<stdin>:1: UserWarning: Parsing dates in DD/MM/YYYY format when dayfirst=False (the default) was specified. This may lead to inconsistently-parsed dates! Specify a format to ensure consistent parsing.
DatetimeIndex(['2000-01-01', '2000-05-31', '2001-05-31', '2000-01-02'], dtype='datetime64[ns]', freq=None)

Like this, the warning would only be emitted once, and (I think) would be a bit clearer

And while I’m at it format should be datetime_format for clarity.

This’d have to go through a deprecation cycle, not sure it’d be worth it

[DrNickBailey]

In your example the wording Parsing '15/01/2006' in DD/MM/YYYY format. Provide format to ensure consistent parsing. still makes no sense to me as a (British/international) user as I already thought ‘11/01/2000’ was in DD/MM/YYYY, so the “warning” is confusing.

I’d also wager that dayfirst as an argument name is unclear – what day is first? dayfirst could mean a number of things to the inexperienced pandas user. monthfirst would be more understandable to the international audience. Though datetime_dayfirst. And while I’m at it format should be datetime_format for clarity.

Sorry for my hate of MM-DD. ISO 8601 is a standard for a reason and solves all ambiguity with dates!