Points can be banked one UY ahead. So, with a Sept UY, your 2011 UY points, when banked, would then be eligible for travel during your 2012 UY, September 1st, 2012 until August 31st, 2013. If not used, those banked points would expire August 31st, 2013
If you borrow points, they become part of the UY you borrow them in to and will expire at the end of that UY. So, if you were traveling in 2012 and needed to borrow 2013 UY points, with a Sept UY, they would then expire August 31st, 2013. You can only borrow points to complete a reservation. Everything goes by travel date and MS will use up any banked points first, then use current UY points, and if you are short, borrow what you need to complete the reservation.
Banking and borrowing are final transactions and can not be changed. So, it is always a good idea to be pretty confident you are traveling before you borrow points.