Actually, the terms state:

"After a second qualifying stay with arrival between 2/28/13 and 5/15/13"

It's right there in post 1 of the main thread about this promo:

http://www.flyertalk.com/forum/choic...00-points.html

It's clear to me that "arrival" means the day you arrive must be by May 15, but you can check out after May 15.