Org mode can automatically record a timestamp and possibly a note when you mark a TODO item as DONE, or even each time you change the state of a TODO item. This system is highly configurable; settings can be on a per-keyword basis and can be localized to a file or even a subtree. For information on how to clock working time for a task, see Clocking work time.
|• Closing items:||When was this entry marked DONE?|
|• Tracking TODO state changes:||When did the status change?|