Before eight months pass in the current year, you can bank any of the current year points. They have to be used by the end of the next use year.
You can borrow one year, use a current year and bank the previous year's points to get up to three years of points to use for a trip. But if your trip starts in one use year and ends in another, you can even use up to four year's worth of points.
They don't automatically "roll over". You have to bank them.