Manually Edit Level Expiration Dates

One big drawback of this plugin is that the Membership starts from the date/time they sign up, but many memberships work from specific set dates & times.

That being said, how do I work around this. I need everyone to move levels on the same date & time?

My situation
Our subscription started on April 17th when those who paid were given the initial Bonus level
Week 1, is supposed to be available to all members on April 24th at 8am.
Week 2 - May 6, 8 am
Week 3 - May 13, 8 am
Week 4 - May 20, 8 am

I can't do any of this automatically with Membership, so how do I do it manually? I can change their access level, but how do I change their expiration date, since they all need to expire at the same day/time.