Right. I've updated my code to be more correct:

instead of:

if not start_time:
    start_time = default_time

it now reads:

if start_time is None:
    start_time = default_time

which operates correctly and works fine for my case, I just thought it was odd that one time out of all of them evaluates to False.  I too wonder if it's by design or not.

It's definitely not documented if it is by design.