37 U.S.C. § 205 — Computation: service creditable

(a) To figure out a uniformed service member's years of service for computing basic pay, add together the following periods (subject to the limits in subsections (b) and (c)): (1) All periods of active service as an officer, Army field clerk, flight officer, aviation midshipman, or enlisted member of a uniformed service. (2) All periods enlisted or appointed as an officer, Army field clerk, or flight officer in any of these: a regular component of a uniformed service; a reserve component; the Space Force; the Regular Army Reserve; the National Guard; the National Guard Reserve; the Naval Militia; the National Naval Volunteers; the Navy Reserve Force; the Army without a specified component; the Air Force without a specified component; the Marine Corps Reserve Force; the Philippine Scouts; or the Philippine Constabulary. (3) For a commissioned officer serving on June 30, 1922: all service that then counted toward longevity pay, plus any full-time service as a contract surgeon. (4) All time as a nurse, reserve nurse, or commissioned officer in the Army Nurse Corps (as it existed before April 16, 1947), the Navy Nurse Corps (same date), the Public Health Service, or a reserve component of any of these. (5) All time as a deck officer or junior engineer in the National Oceanic and Atmospheric Administration — which includes equivalent time in NOAA's predecessor agencies, the Environmental Science Services Administration and the Coast and Geodetic Survey. (6) All periods that were allowed to count toward basic pay under the law in effect on January 10, 1962. (7) All time spent: on a temporary disability retired list, an honorary retired list, or any uniformed service's retired list; entitled to retired pay, retirement pay, or retainer pay from a uniformed service or the VA, as a Fleet Reserve or Fleet Marine Corps Reserve member; or as a member of the Honorary Reserve of the Officers' Reserve Corps or the Organized Reserve Corps. There's a limit: except for active service under clause (1), and except as separate rules in title 10 provide, time spent on a retired list, in retired status, or in the Fleet Reserve/Fleet Marine Corps Reserve — even if it counts under clauses (2) through (7) — cannot be used to increase someone's retired pay, retirement pay, or retainer pay. (b) No period of time can be counted twice under subsection (a). (c) The periods counted under subsection (a) must, under the Secretary's regulations, include service performed before the member turned 18. (d) A commissioned officer cannot count, toward basic pay, any service performed after October 13, 1964 while also serving in the Senior Reserve Officers' Training Corps at the same time — except that service performed on or after August 1, 1979, other than training as an enlisted Selected Reserve member, can be counted. (e) (1) For a member who enlists in a reserve component, certain service described in paragraph (2) cannot be counted under this section. (2) That excluded service is: service in a reserve component under specific enlistment provisions (10 U.S.C. § 12103(b) or (d)) performed before the member starts active duty under that provision — unless the member did inactive-duty training first; and service under another enlistment provision (10 U.S.C. § 513), except for active duty ordered under chapter 1209 of title 10 or another law. (f) For a commissioned officer appointed under 10 U.S.C. § 12203 after getting financial assistance under 10 U.S.C. § 16401, service counted under this section cannot include time after January 1, 2000 spent concurrently as both an enlisted member of the Marine Corps Platoon Leaders Class program and the Marine Corps Reserve — except that service performed before commissioning, while also in the Platoon Leaders Class program, as an enlisted member on active duty or in the Selected Reserve, can still be counted.