Date & time of Completed and status changes

Hi, I would like to set the completed date (and time) for proper administration and evaluation/benchmarking purposes. Currently, I can only set the status without recording the date & time of the change. The date shouldn't be the date when I set it (that would be the default, though), I need the option to change that - e.g. I click 50% today, though that happend yesterday. The minimum feature is to set the date&time for Completed. Full feature is to set that for each status change, even when e.g. a completed is set back to 50% and after some time again to completed. Thanks, Géza