if you have a sept UY and have used up all your 2011 and 2012 pts, you could still call in nov 2011 and book a stay for oct 2012 using borrowed 2013 pts.
remember that when you bank or borrow pts, it is a "final" transaction. your pts move to that new UY and that is where they will expire if not used - they cannot be put back into their original UY or banked into the next UY.
ideally, you never want to travel in the last 3 months or so of your UY (jun-jul-aug for a sept UY) - largely due to traveling after your banking window has closed, which limits your options for any current year pts used to book the stay.
use year month is only an issue if you need to cancel. it has nothing to do with when you can call to book a stay.
might be more info than you want, but here are some examples:
if you have a june UY and a may 2012 trip and you need to cancel 60 days out, your banking window closed on jan 31, 2012 so you'd have to use all of the pts in the reservation before they expire on may 31, 2012.
if you have a june UY and a june 2012 trip and you need to cancel 60 days out, your banking window won't close until jan 31, 2013 so you'd have until then to decide whether to use current year (2012) pts for a reservation in that UY or else bank them to the next year. (you'd still need to use banked and borrowed pts by may 31, 2013.)
if you have a june UY and a may 2012 trip and you need to cancel 5 days out, the pts would turn into "holding" pts so you'd have to use all of the pts in the reservation subject to a 60-day-out limitation - but that won't matter since they expire on may 31...which is less than 60 days out.
if you have a june UY and a june 2012 trip and you need to cancel 5 days out, the pts would turn into "holding" pts so you'd have to use all of the pts in the reservation subject to a 60-day-out limitation - but at least you'd have 12 months before they expire on may 31, 2013 to decide what to do with them.
more info about UY is here if you need it:
http://www.disboards.com/showthread.php?t=1942668