Calculate AGE in years

[Kessie1971]

When i try the Improvement of your class then I get the following erors

3.4.4 (v3.4.4:737efcadf5a6, Dec 20 2015, 20:20:57) [MSC v.1600 64 bit (AMD64)]
Python Type "help", "copyright", "credits" or "license" for more information.
[evaluate Objectgeoriënteerd programmeren_FeedackVraag4a.py]
Traceback (most recent call last):
File "C:\Python\Objectgeoriënteerd programmeren_FeedackVraag4a.py", line 42, in <module>
p1 = Persoon("John", "M", "29-2-1980")
File "C:\Python\Objectgeoriënteerd programmeren_FeedackVraag4a.py", line 5, in <module>
self._geboortedatum = datetime.strptime(geboortedatum, "%d-%m-%Y").date()
builtins.NameError: name 'datetime' is not defined