FWIW, I finally got a response from Tesla Customer Support:
Thank you for contacting Tesla. I have included steps to help resolve your concern.
Scheduled Charging:
When you set a scheduled charging time, Model S displays the set time to begin charging when you are parked at the scheduled location. If, at the scheduled time, Model S is not plugged in at the location, charging starts as soon as you plug it in, provided you plug it in within six hours of the scheduled time.
If plugged in after six hours, charging does not start until the scheduled time on the next day. To override this setting, touch Start Charging or Stop Charging. When you set a scheduled charging time, Model S displays the set time on the instrument panel and touchscreen.
To set up a scheduled departure time, go to Charging > Scheduled Charging > Schedule and follow the onscreen instructions to customize your schedule.