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.