Expiry date calculation based on order completion date

Consider I'm selling video course which is divided in 3 parts. And I'm going to upload these videos one-by-one at interval of 3 weeks. These videos will be available for download for 1 year that means the download link should get expire in 1 year.

I've created a downloadable product & uploaded 1st video & started selling on my site. Some people bought video course. After 3 weeks when I'm going to upload next video, they will get access to that video for 1 year calculated from NOW, but they paid for the video course 3 weeks earlier. 

Why should they get access for 1 year from NOW? Don't you think expiry date should be calculated based on order completion date instead of file upload date?

Your thoughts?