(a) If the time of injury occurs within the first year after the employee has retired, the payrate for compensation purposes shall be one fifty-second part of the employee's average annual earnings during the fifty-two week period preceding retirement.

(b) If the time of injury occurs more than one year after the employee has retired the payrate for compensation purposes shall be the national average weekly wage, determined according to section 6(b)(3) of the Act, 33 U.S.C. 906(b)(3), at the time of injury.