a may 2015 stay falls in your feb 2015 UY (feb 2015 to jan 2016).
so you can use banked 2014 UY pts + current 2015 UY pts + borrowed 2016 UY pts for that stay in your 2015 UY.
so yes, you could have called as early as june 2014 to book may 2015 with borrowed 2016 pts for your home resort. (and yes, you can book it as soon as your 7 month window opens for a non-home resort.)
edited to add: remember that UY is about when you use the pts for a stay. nothing at all to do with when you call to book (that is all based on home resort.)