your UY is about when you are staying. it has nothing to do with when you call to book. nothing.
you can book, at most, 11 months out at your home resort. that is what really limits you as ALL of your pts are already available to you. the other limiting factor is that you can only move pts 1 year forward or back by banking and borrowing, and you cannot move those pts again.
your August 2013 use year pts are valid for stays from aug 1, 2013 to july 31, 2014. if you borrow them, you move them to the 2012 UY, so they would be valid for stays from aug 1, 2012 to july 31, 2013.
so if you were staying on august 1, 2012, the earliest you could have booked your home resort would be sept 1, 2011 (11 months ahead.)
so when you called on sept 1, 2011, you could have booked the stay with banked 2011 pts (if you had any) + current 2012 pts ("current" based on the date of the stay, not when you call) + borrowed 2013 pts (if needed to complete the reservation).