In [7]: from datetime import datetime, timedelta
In [8]: dt = datetime(2018, 4, 9, 13, 37, 0)
In [12]: dt + timedelta(milliseconds=42)
Out[12]: datetime.datetime(2018, 4, 9, 13, 37, 0, 42000)

In [13]: dt + relativedelta(millisecond=42)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-13-8e589d33a46f> in <module>
----> 1 dt + relativedelta(millisecond=42)

TypeError: __init__() got an unexpected keyword argument 'millisecond'

In [14]: dt + relativedelta(milliseconds=42)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-14-d64aff9f46cc> in <module>
----> 1 dt + relativedelta(milliseconds=42)

TypeError: __init__() got an unexpected keyword argument 'milliseconds'