event completion date

Hi

Is there a reason why I can't change a completion date on an event?  The system allows me to change the scheduled date of an event - that's good.  But if I forget to mark an event as completed on the day, the system takes the complete date as whenever I flag it as completed.

Eg:  an event is held on Friday.  For whatever reason, I forget to mark it complete that day, and realize weeks later, on a tuesday.  The complete date for that event becomes the Tuesday, not the Friday when it actually occurred.  This becomes a nuisance when I'm the history of activities, as the event gets out of sequence.