35 ILCS 200/21-35
In counties with less than 3,000,000 inhabitants, when the certified assessed valuations for that portion of overlapping taxing districts lying in another county for the preceding year have not been received by the county clerk by March 1, the county board, upon petition of the county clerk, may by resolution or ordinance adopted on or prior to April 1 of that year, adopt the estimated property tax billing system provided for in this Section for taxes for the preceding year. The resolution or ordinance shall be effective only for the year in which it is adopted.
When authorized by the county board to use the estimated property tax billing system, the county clerk shall estimate the assessed valuations for the other counties in the overlapping taxing districts from which certified assessed valuations for the preceding year have not been received by March 1. The estimated assessed valuations shall, for purposes of computing the first installment tax billing in the current year, be treated in the same manner as certified assessed valuations. Where estimated assessed valuations are used, the first installment billing shall be prepared and mailed on or before May 1.
The county clerk shall make adjustments in the assessments, based on the actual certified assessed valuations later received from the other counties, and such adjustments shall be included in the tax billings for the second installment. A county using the estimated billing system shall complete and mail the adjusted second installment tax billing on or before August 1.
35 ILCS 200/21-35