A contributor may, prior to retirement, purchase service credit for the time during each legislative session spent serving as a member of the legislative assembly while a member of the fund. The contributor shall pay for the service credit an amount equal to the required member contributions and the state contributions for that period of time plus interest as established by the board. 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.