How to create membership level with rolling content availability?

I am setting up a site where there will be weekly blog posts. We would like the most recent post to be free for a week, and then go into the paid membership area. Bascially, the last seven days of content are free, anything older is only available via membership. How would I go about setting this up? I’d like it to be automatic, and not a matter of having to manually move posts into a different, protected category each week.