Please answer a question about the following article about United States Air Force:

During the board process an officer's record is reviewed by a selection board at the Air Force Personnel Center at Randolph Air Force Base in San Antonio, Texas. At the 10 to 11 year mark, captains will take part in a selection board to major. If not selected, they will meet a follow-on board to determine if they will be allowed to remain in the Air Force. Promotion from major to lieutenant colonel is similar and occurs approximately between the thirteen year (for officers who were promoted to major early "below the zone") and the fifteen year mark, where a certain percentage of majors will be selected below zone (i.e., "early"), in zone (i.e., "on time") or above zone (i.e., "late") for promotion to lieutenant colonel. This process will repeat at the 16 year mark (for officers previously promoted early to major and lieutenant colonel) to the 21 year mark for promotion to full colonel.

When does promotion from major to lieutenant colonel in the USAF typically occur?
thirteen year