(1) Subject to the requirements of each retirement system, a member with five or more years of membership service may purchase additional service credit. Members may purchase one full year of additional service for each five full years of membership service in the retirement system. A member eligible to purchase additional service may purchase months of service totaling 11 months or less.

(2) The board will include a member's additional service when calculating the amount of a benefit, but not for determining early or service retirement eligibility, except in the following cases:

(a) additional service purchased into PERS is required to be included when determining a member's early retirement reduction factor; and

(b) additional service purchased into SRS is required to be used when determining a member's retirement eligibility.

(3) A retired member who returns to active membership in the system from which they retired may purchase one-for-five service after at least 12 months of active service. The amount of one-for-five service which may be purchased will be based on the member's total membership service in the member's current retirement system.