40 ILCS 5/8-177

The board shall also ascertain the service rendered by each employee who was employed by the board on June 30, 1935, between the day before the effective date and July 1, 1935, or the date he attained age 65, prior to July 1, 1935, and shall credit him with 5 3/4% of his annual salary during such service with interest thereon at the effective rate to July 1, 1935, or the date he attained age 65, upon the assumption that 1/12 of such 5 3/4% of such annual salary was due at the end of each month of such service.