Regroup by day, then regroup again by the hour - django

Question

I have a dictionary list of items. For example
activities = [
{'start': datetime.datetime(2015, 9, 21, 8, 0), 'end': datetime.datetime(2015, 9, 21, 9, 0),
u'name': u'First Activity', 'description': 'desc of first activity...'},
{'start': datetime.datetime(2015, 9, 21, 9, 0), 'end': datetime.datetime(2015, 9, 21, 10, 0),
u'name': u'Second Activity', 'description': 'desc of next activity...'},
{'start': datetime.datetime(2015, 9, 22, 8, 0), 'end': datetime.datetime(2015, 9, 21, 9, 0),
u'name': u'Third Activity', 'description': 'desc of third activity...'},
{'start': datetime.datetime(2015, 9, 23, 8, 0), 'end': datetime.datetime(2015, 9, 21, 9, 0),
u'name': u'Next Activity', 'description': 'desc of next activity...'},
{'start': datetime.datetime(2015, 9, 23, 9, 0), 'end': datetime.datetime(2015, 9, 21, 10, 0),
u'name': u'Another Activity', 'description': 'desc of another activity...'},
{'start': datetime.datetime(2015, 9, 21, 10, 0), 'end': datetime.datetime(2015, 9, 21, 11, 0),
u'name': u'New Activity', 'description': 'desc of new activity...'}
]
I would like to do a regroup, first by the day, then perform a second regroup by the hour.
I'm having trouble enough just getting a regroup to work by either the day or by hour.
For example:
{% regroup activities by start|date:"l, n/d" as activity_list_by_day %}
{% for session in activity_list_by_day %}
{{ session.grouper }}
{% endfor %}
Isn't working out well. I don't see how to specify that I want the regroup to work by the day.
Is there some way to manipulate my 'start' item for these regroupings? Or might I need to create some extra items to work with?
Edited to add: I see now I can start out by sorting my activities list with:
activities.sort(key=lambda item:item['start'])
This helps tremendously. Actually might have solved my own problem...