Standardize storing of datetime objects in python code

In the code right now, 'datetimes' are stored a couple different ways:
- as python datetime.datetime objects
- as strings of the format '20Jul2016'
- as strings of the format '2016-07-20 21:30:00'

This is sub-optimal.

I vote we store all 'datetimes' as python datetime.datetime objects internally, and then convert them to strings in the appropriate format at the endpoint where they get used