This is how it works in Toodledo, the task manager I am currently using:
- Repeat from due date means that if I set a task to recur every 30 days, for example, it will recur every 30 days since the due date, independently from when I complete it.
- Repeat from completion date means that if I complete the task 3 days after the due date (for example), the task will recur 30 days after I have completed it, not after the due date – or, in this case, 33 days after due date.