The board shall also ascertain the service rendered by each present employee who attained age 65 subsequent to the day before the effective date and prior to July 1, 1923, between the said first named date and the date he attained age 65. Each such employee shall be credited with 5 3/4% of his annual salary during such service, with interest thereon at the effective rate to 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.
40 ILCS 5/8-179