(a) Each member shall receive months of credited service for months served in active military duty not to exceed twenty-four months: Provided, That any employee may purchase as much as an additional twelve months of service for time served in active military duty that otherwise has not been credited, by paying the actuarial reserve lump sum purchase amount within three years after becoming vested.
(b) "Actuarial reserve lump sum purchase amount" means the purchase annuity rate multiplied by the purchase accrued benefit. The purchase annuity rate is the actuarial lump sum annuity factor calculated on a monthly basis based on the following actuarial assumptions: Interest rate of seven and one-half percent; mortality of the 1983 group annuity mortality table, male rates, applied on a unisex basis to all members; if purchase age is under age fifty, a deferred annuity factor with payments commencing at age fifty; and if purchase age is fifty or over, an immediate annuity factor with payments starting at the purchase age. The purchase accrued benefit is two and three-fourths percent times the purchase military service times the purchase average monthly salary. The purchase military service is the amount of military service being purchased by the employee as a fraction of a year up to a one year maximum. The purchase average monthly salary is the final average monthly salary of the employee at the beginning of the month which is three months prior to the purchase month as if the employee terminated employment on that date. The purchase month is the month in which the employee deposits the actuarial reserve lump sum purchase amount into the plan trust fund in full payment of the service being purchased. The purchase age is the attained age of the employee in years and completed months as of the first day of the purchase month.
(c) Members who are eligible to receive credited service for periods of active military duty must substantiate to the retirement board:
(1) That the member has served one or more periods of active duty as substantiated by a federal form DD-214;