35 ILCS 120/4
As soon as practicable after any return is filed, the Department shall examine such return and shall, if necessary, correct such return according to its best judgment and information. If the correction of a return results in an amount of tax that is understated on the taxpayer's return due to a mathematical error, the Department shall notify the taxpayer that the amount of tax in excess of that shown on the return is due and has been assessed. The term "mathematical error" means arithmetic errors or incorrect computations on the return or supporting schedules. No such notice of additional tax due shall be issued on and after each July 1 and January 1 covering gross receipts received during any month or period of time more than 3 years prior to such July 1 and January 1, respectively. Such notice of additional tax due shall not be considered a notice of tax liability nor shall the taxpayer have any right of protest. In the event that the return is corrected for any reason other than a mathematical error, any return so corrected by the Department shall be prima facie correct and shall be prima facie evidence of the correctness of the amount of tax due, as shown therein. In correcting transaction by transaction reporting returns provided for in Section 3 of this Act, it shall be permissible for the Department to show a single corrected return figure for any given period of a calendar month instead of having to correct each transaction by transaction return form individually and having to show a corrected return figure for each of such transaction by transaction return forms. In making a correction of transaction by transaction, monthly or quarterly returns covering a period of 6 months or more, it shall be permissible for the Department to show a single corrected return figure for any given 6-month period.
Instead of requiring the person filing such return to file an amended return, the Department may simply notify him of the correction or corrections it has made.
Proof of such correction by the Department may be made at any hearing before the Department or the Illinois Independent Tax Tribunal or in any legal proceeding by a reproduced copy or computer print-out of the Department's record relating thereto in the name of the Department under the certificate of the Director of Revenue. If reproduced copies of the Department's records are offered as proof of such correction, the Director must certify that those copies are true and exact copies of records on file with the Department. If computer print-outs of the Department's records are offered as proof of such correction, the Director must certify that those computer print-outs are true and exact representations of records properly entered into standard electronic computing equipment, in the regular course of the Department's business, at or reasonably near the time of the occurrence of the facts recorded, from trustworthy and reliable information. Such certified reproduced copy or certified computer print-out shall without further proof, be admitted into evidence before the Department or in any legal proceeding and shall be prima facie proof of the correctness of the amount of tax due, as shown therein.
If the tax computed upon the basis of the gross receipts as fixed by the Department is greater than the amount of tax due under the return or returns as filed, the Department shall (or if the tax or any part thereof that is admitted to be due by a return or returns, whether filed on time or not, is not paid, the Department may) issue the taxpayer a notice of tax liability for the amount of tax claimed by the Department to be due, together with a penalty in an amount determined in accordance with Section 3-3 of the Uniform Penalty and Interest Act. Provided, that if the incorrectness of any return or returns as determined by the Department is due to negligence or fraud, said penalty shall be in an amount determined in accordance with Section 3-5 or Section 3-6 of the Uniform Penalty and Interest Act, as the case may be. If the notice of tax liability is not based on a correction of the taxpayer's return or returns, but is based on the taxpayer's failure to pay all or a part of the tax admitted by his return or returns (whether filed on time or not) to be due, such notice of tax liability shall be prima facie correct and shall be prima facie evidence of the correctness of the amount of tax due, as shown therein.
Proof of such notice of tax liability by the Department may be made at any hearing before the Department or the Illinois Independent Tax Tribunal or in any legal proceeding by a reproduced copy of the Department's record relating thereto in the name of the Department under the certificate of the Director of Revenue. Such reproduced copy shall without further proof, be admitted into evidence before the Department or in any legal proceeding and shall be prima facie proof of the correctness of the amount of tax due, as shown therein.
If the person filing any return dies or becomes a person under legal disability at any time before the Department issues its notice of tax liability, such notice shall be issued to the administrator, executor or other legal representative, as such, of such person.
Except in case of a fraudulent return, or in the case of an amended return (where a notice of tax liability may be issued on or after each January 1 and July 1 for an amended return filed not more than 3 years prior to such January 1 or July 1, respectively), no notice of tax liability shall be issued on and after each January 1 and July 1 covering gross receipts received during any month or period of time more than 3 years prior to such January 1 and July 1, respectively. If, before the expiration of the time prescribed in this Section for the issuance of a notice of tax liability, both the Department and the taxpayer have consented in writing to its issuance after such time, such notice may be issued at any time prior to the expiration of the period agreed upon. The period so agreed upon may be extended by subsequent agreements in writing made before the expiration of the period previously agreed upon. The foregoing limitations upon the issuance of a notice of tax liability shall not apply to the issuance of a notice of tax liability with respect to any period of time prior thereto in cases where the Department has, within the period of limitation then provided, notified the person making the return of a notice of tax liability even though such return, with which the tax that was shown by such return to be due was paid when the return was filed, had not been corrected by the Department in the manner required herein prior to the issuance of such notice, but in no case shall the amount of any such notice of tax liability for any period otherwise barred by this Act exceed for such period the amount shown in the notice of tax liability theretofore issued.
If, when a tax or penalty under this Act becomes due and payable, the person alleged to be liable therefor is out of the State, the notice of tax liability may be issued within the times herein limited after his coming into or return to the State; and if, after the tax or penalty under this Act becomes due and payable, the person alleged to be liable therefor departs from and remains out of the State, the time of his or her absence is no part of the time limited for the issuance of the notice of tax liability; but the foregoing provisions concerning absence from the State shall not apply to any case in which, at the time when a tax or penalty becomes due under this Act, the person allegedly liable therefor is not a resident of this State.
The time limitation period on the Department's right to issue a notice of tax liability shall not run during any period of time in which the Order of any Court has the effect of enjoining or restraining the Department from issuing the notice of tax liability.
If such person or legal representative shall within 60 days after such notice of tax liability file a protest to said notice of tax liability with the Department and request a hearing thereon, the Department shall give notice to such person or legal representative of the time and place fixed for such hearing and shall hold a hearing in conformity with the provisions of this Act, and pursuant thereto shall issue to such person or legal representative a final assessment for the amount found to be due as a result of such hearing. On or after July 1, 2013, protests concerning matters that are subject to the jurisdiction of the Illinois Independent Tax Tribunal shall be filed with the Illinois Independent Tax Tribunal in accordance with the Illinois Independent Tax Tribunal Act of 2012, and hearings concerning those matters shall be held before the Tribunal in accordance with that Act. The Tribunal shall give notice to such person of the time and place fixed for such hearing and shall hold a hearing. With respect to protests filed with the Department prior to July 1, 2013 that would otherwise be subject to the jurisdiction of the Illinois Independent Tax Tribunal, the taxpayer may elect to be subject to the provisions of the Illinois Independent Tax Tribunal Act of 2012 at any time on or after July 1, 2013, but not later than 30 days after the date on which the protest was filed. If made, the election shall be irrevocable.
If a protest to the notice of tax liability and a request for a hearing thereon is not filed within 60 days after such notice, such notice of tax liability shall become final without the necessity of a final assessment being issued and shall be deemed to be a final assessment.
Notwithstanding any other provisions of this Act, any amount paid as tax or in respect of tax paid under this Act, other than amounts paid as quarter-monthly payments, shall be deemed assessed upon the date of receipt of payment.
After the issuance of a final assessment, or a notice of tax liability which becomes final without the necessity of actually issuing a final assessment as hereinbefore provided, the Department, at any time before such assessment is reduced to judgment, may (subject to rules of the Department) grant a rehearing (or grant departmental review and hold an original hearing if no previous hearing in the matter has been held) upon the application of the person aggrieved. Pursuant to such hearing or rehearing, the Department shall issue a revised final assessment to such person or his legal representative for the amount found to be due as a result of such hearing or rehearing.
35 ILCS 120/4