To be clear, your 2015 points can be used for the night of expiration. So, for a February use year, you could use 2015 points with a check-out date of March 1st.
You can bank any leftover current year points. If you borrowed points, just make sure you use those first. Member Services can reallocate points to you reservations to use those first if things get mixed around. You cannot bank points that have already been borrowed.