Illinois Statutes Chapter 40. Pensions § 5/15-113.1. Service for employment with an employer defined under Section 15-106

(a) periods prior to September 1, 1941 during which a person was permanently and continuously employed by an employer.

(b) periods after August 31, 1941 during which a person was an employee except (1) those during which the employee elected not to participate or was ineligible to participate, (2) those during which the employee was on leave of absence at less than 50% pay, except military and disability leave, but failed, in accordance with rules prescribed by the board, to elect to make and to pay the contributions required under Section 15-157, and (3) those during which the employee's eligibility for disability benefit was being considered by the board or reviewed by the courts, if the disability benefit was denied.

(c) periods after August 31, 1941 during which a person was employed at least one-half time for an employer preceding the date of becoming a participant or during which a person was employed at least one-half time for an employer not subject to “The 1941 Act”  1 which employer has since been included as an employer under “The 1941 Act”, or this Article, provided the person makes the contributions required under Section 15-157 based on the rate of earnings during this period equal to the basic compensation on the date of becoming a participating employee together with compound interest from the date participation began to the date payment is received by the board at the rate of 6% per annum through August 31, 1982, and at the effective rates after that date, and provided that the contributions required under Section 15-155 are also made. However, no service credit shall be allowed for any period of employment during which an individual is excluded from the definition of an employee as provided under subsection (b) of Section 15-107.