40 ILCS 5/8-125
"Annuity": Equal monthly payments for life, unless otherwise specified.
For annuities taking effect before January 1, 1998, the first payment shall be due and payable one month after the occurrence of the event upon which payment of the annuity depends, and the last payment shall be due and payable as of the date of the annuitant's death and shall be prorated from the date of the last preceding payment to the date of death for deaths that occur on or before March 31, 2000. All payments made on or after April 1, 2000 shall be made on the first day of the calendar month and the last payment shall be made on the first day of the calendar month in which the annuity payment period ends. All payments for months beginning with April of 2000 shall be for the entire calendar month, without proration. A pro rata amount shall be paid for that part of the month from the March 2000 annuity payment date through March 31, 2000.
For annuities taking effect on or after January 1, 1998, payments shall be made as of the first day of the calendar month, with the first payment to be made as of the first day of the calendar month coincidental with or next following the first day of the annuity payment period, and the last payment to be made as of the first day of the calendar month in which the annuity payment period ends. For annuities taking effect on or after January 1, 1998, all payments shall be for the entire calendar month, without proration. The date on which the annuity payment period begins shall not be prior to termination or more than one year prior to receipt by the board of the written application for benefits.
For the purposes of this Section, the "annuity payment period" means the period beginning on the day after the occurrence of the event upon which payment of the annuity depends, and ending on the day upon which the death of the annuitant or other event terminating the annuity occurs.
40 ILCS 5/8-125