I don't have advice for you, but I can say that there have been similar posts here in the past, and that the T&C's do state that the entire stay and all nights are credited based on the year you checked OUT, not IN.
You might get a supervisor to cut you some slack, but no guarantees.
Also keep in mind for future reference that checking out and checking back in will also not solve the problem since back-to-back stays at the same property count as one stay. You would need to check out and check into a different hotel.