type hint returns NameError: name 'datetime' not defined

Either import datetime and use datetime.datetime as hint, or from datetime import datetime and use datetime as hint.

You need to import datetime, or use a string (remember, it is just an hint).

>>> def f(x: datetime):
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'datetime' is not defined
>>> def f(x: 'datetime'):
...     pass
...
>>>
>>> from datetime import datetime
>>> def f(x: datetime):
...     pass
...
>>>

Python 3.7.4