Duration Dependency

Is there a way to keep a duration even if a start or end date changes? 

For example: I have a task that needs to be completed in 5 days. The initial start date was 8/20 and the end date was 8/24. Due to delays, I start on 8/21 and end on 8/28 (lets just say). 

I still need to show the original duration it had to be completed even though the new duration is 8 days. 

Is there a creative way to keep the original duration with the adjusting dates? Or can I make it a task constraint and try to show the original duration that way, while having the new duration? 

Thank you for any tips or leads.