(a) For the purpose of computing the years of service for pay and allowances of an officer of the Army, Navy, Air Force, or Marine Corps, including retired pay, severance pay, readjustment pay, separation pay, and basic pay, the total years of service of such officer shall be computed by adding to that service so creditable on the day before the effective date of this Act [Sept. 15, 1981] all subsequent service as computed under title 10, United States Code, as amended by this Act.
“(b) An officer of the Army, Navy, Air Force, or Marine Corps who was on active duty on the effective date of this Act [Sept. 15, 1981] and who is retired under section 1251 of title 10, United States Code, as added by this Act, shall be entitled to retired pay in an amount equal to not less than 50 percent of the basic pay upon which his retired pay is based.
“(c) The service that an officer of the Army, Navy, Air Force, or Marine Corps has in a particular grade is the sum of—“(A) the years, months, and days of service in that grade accrued under the laws in effect before the effective date of this Act [Sept. 15, 1981]; and
“(B) the years, months, and days of service in that grade accrued under the laws in effect on and after the effective date of this Act.