Thanks - I think I understand the interaction between "Start time" and "At these hours" better. The Start time would be when the flow is 'eligible' to run, and if no "At these hours" coded, it would simply run then. However, the options of picking days of the week and potentially multiple hours (i.e.; multiple runs per day) are advanced features.
I assume if I had my "Start time" as 2019-02-22T11:00:00 (no Z) and my timezone set to Central US, and Days Mon, Wed, Fri then it would run at 11:00 Central each Mon, Wed, Fri starting 2/22 (a Fri), assuming it was saved before 11:00 on 2/22.
However, if down the road I want to change it to run 2x daily or change the run time, I could simply use the "At these hours" field to make the updates.
Thanks for your help!