35 ILCS 200/21-20
Except as otherwise provided in Section 21-40, in counties with less than 3,000,000 inhabitants in which the accelerated method of billing and paying taxes provided for in Section 21-30 is in effect, the estimated first installment of unpaid taxes shall be deemed delinquent and shall bear interest after a date not later than June 1 annually as provided for in the ordinance or resolution of the county board adopting the accelerated method, at the rate of 1 1/2% per month or portion thereof until paid or forfeited. The second installment of unpaid taxes shall be deemed delinquent and shall bear interest after August 1 annually at the same interest rate until paid or forfeited. Payment received by mail and postmarked on or before the required due date is not delinquent. Notwithstanding any other provision of law, if a taxpayer owes an arrearage of taxes due to an administrative error, and if the county collector sends a separate bill for that arrearage as provided in Section 14-41, then any part of the arrearage of taxes that remains unpaid on the day after the due date specified on that tax bill shall be deemed delinquent and shall bear interest after that date at the rate of 1 1/2% per month or portion thereof.
If an Illinois resident who is a member of the Illinois National Guard or a reserve component of the armed forces of the United States and who has an ownership interest in property taxed under this Act is called to active duty for deployment outside the continental United States and is on active duty on the due date of any installment of taxes due under this Act, he or she shall not be deemed delinquent in the payment of the installment and no interest shall accrue or be charged as a penalty on the installment until 180 days after that member returns from active duty. To be deemed not delinquent in the payment of an installment of taxes and any interest on that installment, the reservist or guardsperson must make a reasonable effort to notify the county clerk and the county collector of his or her activation to active duty and must notify the county clerk and the county collector within 180 days after his or her deactivation and provide verification of the date of his or her deactivation. An installment of property taxes on the property of any reservist or guardsperson who fails to provide timely notice and verification of deactivation to the county clerk is subject to interest and penalties as delinquent taxes under this Code from the date of deactivation.
35 ILCS 200/21-20