Fix coord_categorisation.add_weekday (#4139)

* fix add_weekday

* fix black link

docs/src/developers_guide/contributing_code_formatting.rst

@@ -39,7 +39,7 @@ linted according to our ``.flake8`` configuration file. Note that,
 for each ``.pre-commit-config.yaml`` git hook.
 
 Additionally, you may wish to enable ``black`` for your preferred
-`editor/IDE <https://black.readthedocs.io/en/stable/editor_integration.html#editor-integration>`_.
+`editor/IDE <https://black.readthedocs.io/en/stable/integrations/editors.html>`_.
 
 With the ``pre-commit`` configured, the output of performing a ``git commit``
 will look similar to::

lib/iris/coord_categorisation.py

@@ -93,7 +93,7 @@ def vectorised_fn(*args):
 # Private "helper" function
 def _pt_date(coord, time):
     """
-    Return the date of a time-coordinate point.
+    Return the datetime of a time-coordinate point.
 
     Args:
 
@@ -103,14 +103,11 @@ def _pt_date(coord, time):
         value of a coordinate point
 
     Returns:
-        datetime.date
+        cftime.datetime
     """
     # NOTE: All of the currently defined categorisation functions are
     # calendar operations on Time coordinates.
-    #  - All these currently depend on Unit::num2date, which is deprecated (!!)
-    #  - We will want to do better, when we sort out our own Calendars.
-    #  - For now, just make sure these all call through this one function.
-    return coord.units.num2date(time)
+    return coord.units.num2date(time, only_use_cftime_datetimes=True)
 
 
 # --------------------------------------------