35 ILCS 615/3
Except as provided in this Section, on or before the 15th day of each month, each taxpayer shall make a return to the Department for the preceding calendar month, stating:
In making such return the taxpayer may use any reasonable method to derive reportable "therms" and "gross receipts" from his billing and payment records.
Any taxpayer required to make payments under this Section may make the payments by electronic funds transfer. The Department shall adopt rules necessary to effectuate a program of electronic funds transfer.
If the taxpayer's average monthly tax liability to the Department does not exceed $100.00, the Department may authorize his returns to be filed on a quarter annual basis, with the return for January, February and March of a given year being due by April 30 of such year; with the return for April, May and June of a given year being due by July 31 of such year; with the return for July, August and September of a given year being due by October 31 of such year, and with the return for October, November and December of a given year being due by January 31 of the following year.
If the taxpayer's average monthly tax liability to the Department does not exceed $20.00, the Department may authorize his returns to be filed on an annual basis, with the return for a given year being due by January 31 of the following year.
Such quarter annual and annual returns, as to form and substance, shall be subject to the same requirements as monthly returns.
Notwithstanding any other provision in this Act concerning the time within which a taxpayer may file his return, in the case of any taxpayer who ceases to engage in a kind of business which makes him responsible for filing returns under this Act, such taxpayer shall file a final return under this Act with the Department not more than one month after discontinuing such business.
In making such return the taxpayer shall determine the value of any reportable consideration other than money received by him and shall include such value in his return. Such determination shall be subject to review and revision by the Department in the same manner as is provided in this Act for the correction of returns.
Each taxpayer whose average monthly liability to the Department under this Act was $10,000 or more during the preceding calendar year, excluding the month of highest liability and the month of lowest liability in such calendar year, and who is not operated by a unit of local government, shall make estimated payments to the Department on or before the 7th, 15th, 22nd and last day of the month during which tax liability to the Department is incurred in an amount not less than the lower of either 22.5% of the taxpayer's actual tax liability for the month or 25% of the taxpayer's actual tax liability for the same calendar month of the preceding year. The amount of such quarter monthly payments shall be credited against the final tax liability of the taxpayer's return for that month. Any outstanding credit, approved by the Department, arising from the taxpayer's overpayment of its final tax liability for any month may be applied to reduce the amount of any subsequent quarter monthly payment or credited against the final tax liability of the taxpayer's return for any subsequent month. If any quarter monthly payment is not paid at the time or in the amount required by this Section, the taxpayer shall be liable for penalty and interest on the difference between the minimum amount due as a payment and the amount of such payment actually and timely paid, except insofar as the taxpayer has previously made payments for that month to the Department in excess of the minimum payments previously due.
If the Director finds that the information required for the making of an accurate return cannot reasonably be compiled by a taxpayer within 15 days after the close of the calendar month for which a return is to be made, he may grant an extension of time for the filing of such return for a period of not to exceed 31 calendar days. The granting of such an extension may be conditioned upon the deposit by the taxpayer with the Department of an amount of money not exceeding the amount estimated by the Director to be due with the return so extended. All such deposits, including any made before the effective date of this amendatory Act of 1975 with the Department, shall be credited against the taxpayer's liabilities under this Act. If any such deposit exceeds the taxpayer's present and probable future liabilities under this Act, the Department shall issue to the taxpayer a credit memorandum, which may be assigned by the taxpayer to a similar taxpayer under this Act, in accordance with reasonable rules and regulations to be prescribed by the Department.
The taxpayer making the return provided for in this Section shall, at the time of making such return, pay to the Department the amount of tax imposed by this Act. All moneys received by the Department under this Act shall be paid into the General Revenue Fund in the State Treasury, except as otherwise provided.
If any payment provided for in this Section exceeds the taxpayer's liabilities under this Act, as shown on an original return, the Department may authorize the taxpayer to credit such excess payment against liability subsequently to be remitted to the Department under this Act, in accordance with reasonable rules adopted by the Department.
35 ILCS 615/3