Is there a reason I can't think off that there is only a start time for charging and no stop time? I assume USA also has similar electricity tariffs with off peak cheap rates. Would make life simpler, I'm pretty sure most EV owners have moved to a tariff where they can optomise costs.
Why not just provide the Tesla with your home charger tariffs and have the machine figure it out based on business rules (time of departure @ SoC) the best strategy for charging?
Much like the Jedlix app is now doing for me.