10 U.S.C. § 630

The Secretary of the military department concerned, under regulations prescribed by the Secretary of Defense— (1) may discharge any officer on the active-duty list who— (A) has less than six years of active commissioned service; or (B) is serving in the grade of second lieutenant or ensign and has been found not qualified for promotion to the grade of first lieutenant or lieutenant (junior grade); and (2) shall, unless the officer has been promoted, discharge any officer described in paragraph (1)(B) at the end of the 18-month period beginning on the date on which the officer is first found not qualified for promotion.