40 ILCS 5/5-122

"Prior Service Annuity" shall be credited for present employees for service rendered prior to the effective date in accordance with the provisions of "Policemen's Annuity and Benefit Fund Act of the Illinois Municipal Code" and this Article. Each such credit shall be improved by interest during the time thereafter the employee is in service until his annuity is fixed. In determining such annuity, the annual salary for the entire period of the employee's service prior to the effective date shall be the salary in effect on the effective date.