At the end of the month, or the year, sometimes I can't remember when I did something. I had a task (a todo) that I finished, but I can't remember exactly when I did it. Sometimes this is important. When did I send the check, when did I paint the walls, when did I finish the work, etc.

The date of the task might not be the date that I finished it on. I have several overdue items in my todo list. Eventually I finish them and mark them as "done". I would like that the date you tag something as "done" automatically be recorded in a "date completed" field for that task. I know that you can sort items by "last updated", and this almost works, except that you have to be careful to not change anything on that task ever again. if you update an item that is already marked with "done", then you lose the date that the task was completed.