40 ILCS 5/6-173

The board shall estimate the amounts required each year to be contributed by the city for all annuities, benefits and administrative expenses. All amounts shall be paid annually by the city into the fund from the taxes herein authorized. If it is not possible or practicable for the city to make contributions for age and service and widow's annuity at the time deductions from employees' salaries are made for these purposes, the city shall make such contributions as soon as possible thereafter, with interest thereon at the applicable rate to the time they shall be made.