(a)
Under regulations to be prescribed by the Secretary of the Army or the Secretary of the Air Force, as the case may be, each company, battery, squadron, and detachment of the National Guard, unless excused by the Secretary concerned, shall—
(1)
assemble for drill and instruction, including indoor target practice, at least 48 times each year; and
(2)
participate in training at encampments, maneuvers, outdoor target practice, or other exercises, at least 15 days each year.
However, no member of such unit who has served on active duty for one year or longer shall be required to participate in such training if the first day of such training period falls during the last one hundred and twenty days of his required membership in the National Guard.