40 ILCS 5/8-128

A "Prior Service Annuity" shall be credited to present employees in accordance with the 1921 Act for service rendered prior to the effective date. Each such credit shall be improved by interest at the effective rate during the time the employee is in service until his annuity is fixed. In determining such credit, the employee's annual salary for his entire period of prior service shall be the salary in effect on the effective date.