BUG: to_datetime()'s origin does not use unit specified in the parameter

[ytchanglp]

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

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Code Sample, a copy-pastable example

pd.to_datetime(1, unit='s')                    
# Timestamp('1970-01-01 00:00:01')               

pd.to_datetime(1, unit='s', origin=1)          
# Current --> Timestamp('1970-01-01 00:00:01')               
# Expected -> Timestamp('1970-01-01 00:00:02')       

pd.to_datetime(1, unit='s', origin=1000000000) 
# Current --> Timestamp('1970-01-01 00:00:02')               
# Expected -> Timestamp('2001-09-09 01:46:41')

Problem description

The numeric values of origin should be parsed as number of units defined by unit

Output of pd.show_versions()

INSTALLED VERSIONS

commit : f00ed8f
python : 3.9.6.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19042
machine : AMD64
processor : Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United States.950

pandas : 1.3.0
numpy : 1.21.0
pytz : 2021.1
dateutil : 2.8.1
pip : 21.1.3
setuptools : 57.0.0
Cython : 0.29.23
pytest : 6.2.4
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.6.3
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.1
IPython : 7.25.0
pandas_datareader: None
bs4 : None
bottleneck : 1.3.2
fsspec : 2021.06.1
fastparquet : None
gcsfs : None
matplotlib : 3.4.2
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.7
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : 1.7.0
sqlalchemy : None
tables : None
tabulate : 0.8.9
xarray : None
xlrd : 2.0.1
xlwt : None
numba : 0.53.1

[stephan-hesselmann-by]

This should be easy to fix, but I'm not sure what the correct expected behavior actually is. According to the docs (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.to_datetime.html) origin may be given as "unix" (or POSIX) time. POSIX time is defined in seconds. So should the correct behavior be to use seconds as unit for numeric origin (potentially different from the unit of arg)? Or should origin be in the unit of arg? The current behavior also makes sense since it corresponds to the default Timestamp(origin) conversion.

[ytchanglp]

the doc is very clearly.

The numeric values would be parsed as number of units (defined by unit) since this reference date.

[Dinkarkumar]

Can I work on this issue ?

[stephan-hesselmann-by]

the doc is very clearly.
The numeric values would be parsed as number of units (defined by unit) since this reference date.

At least to me this does not clarify if the reference date itself should be parsed as number of units as well. Especially with Unix time this would not really make sense, because Unix time is per definition always in seconds.