(a) Each increase in rates of basic pay granted, pursuant to a wage survey, to prevailing rate employees is effective not later than the first day of the first pay period which begins on or after the 45th day, excluding Saturdays and Sundays, following the date the wage survey is ordered to be made.

(b) Retroactive pay is payable by reason of an increase in rates of basic pay referred to in subsection (a) of this section only when—

(1) the individual is in the service of the Government of the United States, including service in the armed forces, or the government of the District of Columbia on the date of the issuance of the order granting the increase; or

(2) the individual retired or died during the period beginning on the effective date of the increase and ending on the date of issuance of the order granting the increase, and only for services performed during that period.

For the purpose of this subsection, service in the armed forces includes the period provided by statute for the mandatory restoration of the individual to a position in or under the Government of the United States or the government of the District of Columbia after he is relieved from training and service in the armed forces or discharged from hospitalization following that training and service.