The footprint of the trip that spills into Feb will count as reserve days. All of the credit from the trip will go against your reserve guarantee- in other words, if there is very little flying on the days in Jan, you are working those days at a very steep discount compared to what you would earn in Jan if the Jan days blocked 5:15 or if the trip ended in Jan, or if the trip had no credit.

It basically is a very bad deal. Happened to me in Nov/Dec. Avoid it or swap trips.