If a project is still in progress, simply submit deliverables through the tasks and/or mark tasks as complete as you go. If the project finished ahead of schedule, do the following:
Rename the tasks which finished ahead of schedule, adding “Finished Ahead of Schedule” to their existing title.
Change the duration of the tasks to represent when they were actually completed.
Manually add new one-day-long tasks to the timeline in the spots where we originally promised they’d be done. Name these new tasks “Original Deadline”.
You now have visual documentation of the original deadline, and the new final delivery dates.