37 U.S.C. § 905. — Reserve officers of the Navy or Marine Corps not on the active-duty list: effective date of pay and allowances

((a)) A reserve officer who is promoted under chapter 1405 of title 10 to a grade above lieutenant (junior grade) in the Navy Reserve or above first lieutenant in the Marine Corps Reserve is entitled to the pay and allowances of the grade to which promoted for duty performed from the date on which he becomes eligible for promotion to that grade.

((b)) A reserve officer who is promoted under to the grade of lieutenant (junior grade) in the Navy Reserve or first lieutenant in the Marine Corps Reserve is entitled to the pay and allowances of the higher grade for duty performed from the date given him as his date of rank.