40 ILCS 5/5-190

To require each policeman, including those on vacation and on leave of absence, to file a statement, in such form as the Board directs, concerning service rendered prior to the effective date, from which the Board shall make a determination of the length of such service; to determine, from such information as shall be available, the period of service rendered prior to the effective date by any policeman who fails to file such a statement. Any such determination by the Board shall be conclusive as to any such period of service unless the Board reconsiders and changes the determination.