40 ILCS 5/12-160
To determine the length of service of each present employee rendered prior to the date when he comes under the provisions of "The 1919 Act" or this Article, including all service rendered to any employer as defined herein; to require each employee to file with the board a detailed statement of all such service rendered by him and to prescribe rules and regulations for the filing of such statements; to fix the period for which such employee shall receive credit for prior service from such information as is available, if the employee fails to file the aforesaid statement, or if the board is unable to verify such statement; to certify such statement and issue a certificate to the employee stating the length of prior service allowed.
Such certificate shall be final and conclusive as to length of prior service and amount of credit unless modified by the board, either of its own volition or upon application of the employee, within one year from the date when the certificate or a modified certificate is issued.
All leaves of absence without pay shall be excluded in computing prior service, and leaves of absence on full or part pay shall be included in computing the prior service.
40 ILCS 5/12-160