The insurance is based on your points, so it goes by use year. If you are using points from more than one use year for a trip then you need to buy insurance for both use years. If the points for your trip are from one use year, even if the trip is Dec. to Jan, then you only need to buy insurance for one use year.
Travelex is a good deal for us because it is cheap, it covers all our trips from a use year's points for just one premium, it covers airfare (and we always fly), it is not priced by age like some
travel insurance (and we are OLD), and if you buy it when dues are due in January then the pre-existing condition clause is waived. That last one is a big deal for us because DH has so many health issues and travel insurers can often use pre-existing condition clauses to avoid paying claims. I wish that Travelex would pay for anyone using our points because we give about half our points away every year, but I don't think it does. We have only filed a claim once for airfare but it was for 2 first class tickets and Travelex paid with no problem (after we had a doctor fill our paperwork). DH was hospitalized for a few days during our trip last month but he was discharged in time to make our flight home so we didn't have to file a claim. Still it was comforting to know that Travelex would have helped with costs of changing plane tickets and for costs for me if we had been forced to extend our stay.