(a) If the Tax Collector determines that a taxpayer has failed to pay or has underpaid a tax, that an operator has failed to collect and remit all of a third-party tax, or that a person other than the taxpayer is jointly and severally liable for any unpaid or underpaid tax, including third-party taxes, the Tax Collector may compute and determine any tax deficiency upon the basis of the return or returns or upon the basis of any other information within the Tax Collector's possession or that may come into the Tax Collector's possession. One or more deficiency determinations of the amount due may be made for one or for more than one period.
(b) The amount of the determination, exclusive of penalties, shall bear interest at the rate of 1 percent per month, or fraction thereof, from the 15th day after the close of the month or the monthly period for third-party taxes, or from the last day of February following the close of the annual period, for which the amount or any portion thereof should have been remitted until the date of payment in full, or, in the case of stadium operator admission taxes, from the due dates of said tax as set forth in Article 11, Section 804.
(c) In making a determination, the Tax Collector may offset overpayments for a period or periods together with interest on the overpayments, against underpayments for another period or periods, against penalties, and against the interest on the underpayments. The interest on underpayments and overpayments shall be computed in the manner set forth in Section 6.17-1 for underpayments and in Section 6.15-2 for overpayments.
(d) This Section 6.11-1 shall apply as follows:
(1) to all returns and payments for tax periods ending and taxable events occurring on or before December 31, 2020, except as provided in subsections (d)(2) and (d)(3) of this Section 6.11-1;
(2) to all returns and payments for registration years ending on or before June 30, 2021 with respect to the registration fee imposed under Section 855 of Article 12; and
(3) to all returns and payments for fiscal years ending on or before June 30, 2021 with respect to fees, assessments, and other charges subject to this Article 6 that are imposed on a fiscal year basis.