Dec UY:
Points are good for stays from Dec 1, 2010 - Nov 30, 2011. You have through July 31, 2011 to bank your points into the next UY. If points are banked by July 31, they are good for stays from Dec 1, 2011 - Nov 30, 2012. Once banked, points cannot be banked again.
You can use the scenario above and think through different UY options. UY, IMHO, becomes much more important if you only travel once a year and bank points. For example, we had an Aug UY. When we banked points, those points expired at the end of July the following year. So even though we wanted to travel again in Aug the following year, I couldn't, or I would lose those banked points. It wasn't the end of the world, it was just a nuisance given when we travel.