[Ken Case]

Our current assumption is that the due date is the most important date, so if you have both a start date and a due date the repeat interval is applied to the due date (and the start date is calculated as a lead time relative to that due date).

So, if I have a task with these settings:

start: 11/26, due: 11/28, repeat: every 3 days from completion

and I complete the task on 12/5 (today), I'll get:

start: 12/6 (not yet available), due: 12/8

The new due date (12/8) is the completion date plus three days, the new start date (12/6) is the new due date minus two days (because the old start date was set to two days before the old due date).

Isn't that what you're trying to do?

If you look in the repeat inspector, it will show you the Next Start and Next Due dates (assuming you completed the task immediately); you can check those to make sure you're getting the settings you want (without having to actually check off your task to test it).

P.S. — If you don't specify a due date, then the repeat interval refers to the start date: so if you've set a task to repeat one day after completion and it only has a start date, the new start date will be tomorrow.