40 ILCS 5/8-204

To require each employee to file a statement concerning service rendered the employer, the Retirement Board, and the Board of Trustees of any municipal pension fund as defined in this Article, prior to the effective date. The board shall make a determination of the length of such service and establish from any available information, the period of service rendered prior to the effective date. Such determination shall be conclusive unless the board reconsiders and changes its determination.