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Reproducible Example

In [1]: to_datetime('15000101', format='%Y%m%d', errors='ignore')
Out[1]: datetime.datetime(1500, 1, 1, 0, 0)

In [2]: to_datetime('1500-01-01', format='%Y-%m-%d', errors='ignore')
Out[2]: '1500-01-01'

Issue Description

It converted to datetime.datetime instead of returning the input

Expected Behavior

'15000101'

Returning the input is what we do for literally every single other format

Installed Versions

INSTALLED VERSIONS ------------------ commit : 8dab54d6573f7186ff0c3b6364d5e4dd635ff3e7 python : 3.8.15.final.0 python-bits : 64 OS : Linux OS-release : 5.10.102.1-microsoft-standard-WSL2 Version : #1 SMP Wed Mar 2 00:30:59 UTC 2022 machine : x86_64 processor : x86_64 byteorder : little LC_ALL : None LANG : en_GB.UTF-8 LOCALE : en_GB.UTF-8 pandas : 1.5.2 numpy : 1.23.5 pytz : 2022.6 dateutil : 2.8.2 setuptools : 65.5.1 pip : 22.3.1 Cython : None pytest : None hypothesis : None sphinx : None blosc : None feather : None xlsxwriter : None lxml.etree : None html5lib : None pymysql : None psycopg2 : None jinja2 : 3.1.2 IPython : 8.6.0 pandas_datareader: None bs4 : 4.11.1 bottleneck : None brotli : None fastparquet : None fsspec : None gcsfs : None matplotlib : None numba : None numexpr : None odfpy : None openpyxl : None pandas_gbq : None pyarrow : None pyreadstat : None pyxlsb : None s3fs : None scipy : None snappy : None sqlalchemy : None tables : None tabulate : None xarray : None xlrd : None xlwt : None zstandard : None tzdata : None None

Comment From: MarcoGorelli

dupe https://github.com/pandas-dev/pandas/issues/14487