I would appreciate to have an option to mark tasks completed and choose a completion date in the past. A possible feature could be a kind of pop-up window which opens by long-pressing the completion-button. In the window there might be an option “yesterday” and the calendar to select an other date in the past.
In my case I often use recurring tasks. Lots of them have due dates “recurring x days after completion”.
Also often I get my self not managed to mark all task completed directly after they are. So instead I mark them completed one or more days later. And that causes my problem. I can not set a completion date in the past. And by this the new due date of the recurring task does not exactly have the due date it should have.