Nights can only roll over into the following year. So you'd roll over 90 to next year, then 0 to the following year. The rollover begins once you've reached the official qualification threshold, so once you reach 60 nights next year, you'd then begin to accumulate rollover nights that would count towards the following year. At least that's how I understand it though it's somewhat unclear. Also does anyone know if there's somewhere to check the total number of nights that you're on track to have rolled over?