An employee of a city having a pension fund under this chapter may, prior to retirement, purchase service for inclusion in the period of service required under this chapter for eligibility for retirement the time during each legislative session spent serving as a member of the legislative assembly while a member of the pension fund. The employee shall pay for this service an amount equal to the required member assessments and employer contributions plus    interest as established by the board of trustees. Service credit for legislative sessions prior to July 1, 1985, must be purchased before January 1, 1986. Service credit for each later legislative session must be purchased within one year after the adjournment of that legislative session.