Keep original (planned) due date when due date was change
When changing due dates, there should be a prompt if user want to track changes. If user said yes, The original due date should be tracked.
Keeping the original due dates will keep track of how many days a task was ovedue before it gets completed.
It should show the original due date and the actual date it was completed and show number of days it was overdue (negative numbers means it was completed before due dates
In overall, it will show the difference of planned project timeline against actual project timeline
Some due date changes are legitimate error and this is why the system should prompt if the change being made needs tracking or the due date was entered incorrectly and the change should not be tracked.
I would prefer if the planner is asked to approve the due date change, rather than asking the assignee whether she wishes to track the change. BY default, the change should be tracked.