It is our understanding that Adobe defines subscriptions as a 1-year time period where subscribers get all the issues publishied within in it. Does that include paid supplements that we put out within that time, or are subscribers entitled to only the 12 issues that the subscription stipulates?
Subscriptions last as long as you establish. Apple owns the timeframe. That said, it is indeed a timeframe. A 3-month subscription bought June 1 will expire Sept 1, and any document therein will appear as entitled.
A "your mileage may vary" hack that some publishers have done is to drastically pre-date or post-date folios. This will cause them to appear at the top (post-date) or bottom (pre-date) of Viewer App's Library (new == first), as you choose, and they're guaranteed to fall out of your subscription range. A folio with a publish date of 2014, for example, would not appear as entitled to those with any subscription smaller than 2 years or so. This really only works for limited-run publications, because they'll eventually become available when they fall into a subscriber's range.
My suggestion is a hack where you're gaming Apple's date-driven subscription logic. I'd test it out in a development/sandbox app before bestowing it on users.