December UY must be banked by 8 months, end of month- so your unused points must be banked by July 31st. This is the same year to year, regardless of how many points.
If you transfer another members points into your account, their UY remains- so add the 8 months onto their UY to know when those transferred points expire (knowing transferred points may be banked a year).
Of course, as always, more senior members may correct me if I'm inaccurate, as I always heed more experienced advice with gratitude!