docs: to_datetime: document that %Y zero-pads years

[MLpranav]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl
import pandas as pd
import datetime

print(pl.Series(['12-AUG-20']).str.strptime(pl.Date, '%d-%b-%Y')[0])
# datetime.date(20, 8, 12) ❌

print(pd.to_datetime('12-AUG-20', format='%d-%b-%Y'))
# ValueError ✅

print(datetime.datetime.strptime('12-AUG-20', '%d-%b-%Y'))
# ValueError ✅

###

print(pl.Series(['12-AUG-20']).str.strptime(pl.Date, '%d-%b-%y')[0])
# datetime.date(2020, 8, 12) ✅

print(pd.to_datetime('12-AUG-20', format='%d-%b-%y'))
# Timestamp('2020-08-12 00:00:00') ✅

print(datetime.datetime.strptime('12-AUG-20', '%d-%b-%y'))
# datetime.datetime(2020, 8, 12, 0, 0) ✅

Log output

No response

Issue description

Instead of throwing an error when a 2 digit year is passed for format code %Y (not %y), polars is automatically zero padding it.

(By silently ignoring this exception, a try except clause for handling wrongly formatted data is not getting triggered.)

Expected behavior

An exception should be raised, similar to pandas' and datetime's behaviour.

Installed versions

--------Version info---------
Polars:              1.7.1
Index type:          UInt32
Platform:            macOS-14.6.1-arm64-arm-64bit
Python:              3.12.4 (main, Jun  6 2024, 18:26:44) [Clang 15.0.0 (clang-1500.3.9.4)]

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
cloudpickle          3.0.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               <not installed>
gevent               24.2.1
great_tables         <not installed>
matplotlib           3.9.1
nest_asyncio         1.6.0
numpy                1.26.4
openpyxl             3.1.2
pandas               2.2.2
pyarrow              17.0.0
pydantic             2.8.2
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           3.2.0

[MarcoGorelli]

thanks @MLpranav for the report

chrono parses it like this

use chrono::NaiveDate;
fn main() {
    let res = NaiveDate::parse_from_str("12-AUG-20", "%d-%b-%Y");
    println!("{:?}", res);  // Ok(0020-08-12)
}

but maybe it should be documented

[MLpranav]

I would still consider this a bug, as %Y is not supposed to match 2 digit years, especially when default values of "strict" and "exact" are set to True. This is also not in line with the default pandas/datetime behaviour.