40 ILCS 5/14-104.2
Any member who participated in a voluntary furlough plan or who was subject to a 4 day work week pursuant to negotiated agreements in fiscal years 1983 and 1984 may receive service and earnings credit for such periods by making contributions on or before December 31, 1984, based on the rate of compensation in effect immediately prior to the furlough or the fifth work day of any calendar week and the contribution rate then in effect. Contributions made under this Section must be made prior to retirement except that any member who retired on or before August 22, 1983 may receive service and earnings credit for such periods by making the contribution as required in this Section. Any annuitant who establishes service and earnings credit as herein provided shall have his retirement annuity adjusted retroactively to the date of retirement.
40 ILCS 5/14-104.2