[X] I have checked that this issue has not already been reported.
[X] I have confirmed this bug exists on the latest version of pandas.
[X] I have confirmed this bug exists on the main branch of pandas.
>>> import pandas as pd
>>> pd.Timestamp(str(pd.Timestamp("0001-01-01")))
Timestamp('2001-01-01 00:00:00')
>>> pd.Timestamp("0001-01-01").isoformat()
'1-01-01T00:00:00'
With https://github.com/pandas-dev/pandas/pull/49737 it is now possible to create Timestamp objects without four-digit years. The default and ISO formatted versions of them do not contain zero-padded years, which can lead to ambiguous date strings (https://github.com/pandas-dev/pandas/issues/37071) and improper round tripping between str and Timestamp as illustrated in the example.
cc: @keewis
>>> import pandas as pd
>>> pd.Timestamp(str(pd.Timestamp("0001-01-01")))
Timestamp('0001-01-01 00:00:00')
>>> pd.Timestamp("0001-01-01").isoformat()
'0001-01-01T00:00:00'
commit : d3f0e9a95d220cd349fbc2d9b107ebebc5d7b58a python : 3.8.5.final.0 python-bits : 64 OS : Darwin OS-release : 22.2.0 Version : Darwin Kernel Version 22.2.0: Fri Nov 11 02:08:47 PST 2022; root:xnu-8792.61.2~4/RELEASE_X86_64 machine : x86_64 processor : i386 byteorder : little LC_ALL : None LANG : en_US.UTF-8 LOCALE : en_US.UTF-8
pandas : 1.5.0.dev0+2555.gd3f0e9a95d numpy : 1.22.0.dev0+756.g5350aa097 pytz : 2020.1 dateutil : 2.8.2 setuptools : 49.2.0.post20200712 pip : 20.1.1 Cython : None pytest : 5.4.3 hypothesis : 5.22.0 sphinx : None blosc : None feather : None xlsxwriter : None lxml.etree : 4.5.2 html5lib : None pymysql : None psycopg2 : None jinja2 : 2.11.2 IPython : None pandas_datareader: None bs4 : 4.9.1 bottleneck : 1.4.0.dev0+78.g7c685ae brotli : fastparquet : None fsspec : 0.7.4 gcsfs : None matplotlib : 3.4.3 numba : 0.50.1 numexpr : None odfpy : None openpyxl : None pandas_gbq : None pyarrow : None pyreadstat : None pyxlsb : None s3fs : None scipy : 1.8.0.dev0+1577.2f60c33 snappy : None sqlalchemy : None tables : None tabulate : None xarray : 0.20.3.dev517+g17933e76 xlrd : None zstandard : None tzdata : None qtpy : None pyqt5 : None
