A member may purchase service credit for the time during each legislative session spent serving as a member of the legislative assembly while holding eligible employment under this    chapter. The member shall pay for this service credit an amount equal to the required member assessments and employer contributions 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.