When is the reminder sent?

The reminder is the only flow that is scheduled rather than sent when something happens. You choose the timing under Settings → Messages.

The options are fixed — the day before, two hours before, three days before — rather than a free field for minutes. "90" and "9000" look alike in a number field, and you have to do the arithmetic yourself to arrive at "the day before".

The other messages are sent when they happen: the confirmation when the appointment is created, the cancellation when it is cancelled. A delay on a confirmation would be a confirmation that arrived too late.

The recall message also has a timing, but it lives on the service — the interval differs from treatment to treatment.