Percent Complete

Is there a way to keep task %complete value when changing the end date for a deliverable. For example, if I have a task that is 50% complete. When I change the end date, the percent complete gets recalculated proportionally based on how far out I move the end date.

I would like to keep the %complete the same and not have to re-enter each time as not all deliverables are time based to determine completion.