40 ILCS 5/14-104.9
A member may establish creditable service under this Article for up to one year during which he or she was on a leave of absence from employment for which credit is not otherwise available under this Code, subject to the following conditions: (1) the leave of absence terminated before January 1, 1971, and (2) on or before March 1, 1993, the member files a written application with the System and contributes to the System an amount determined by the Board, equal to (i) employee contributions at the appropriate rate in effect for members of this System during the period for which credit is being established, and based upon the compensation received by the applicant at the time the leave began, plus (ii) the employer's share of the normal cost of the creditable service being established, plus (iii) regular interest, compounded annually, from the date of service to the date of payment.
40 ILCS 5/14-104.9