Special rules relating to estimated tax.

(1) Any amount paid as estimated tax shall be deemed assessed upon the due date for the taxpayer’s return for the taxable year, determined without regard to any extensions of time for filing such return.

(2) No interest or penalty shall be due or paid with respect to a failure to pay estimated taxes except the following:

(a) Except as provided in paragraph (d), the taxpayer shall be liable for interest at the rate of 12 percent per year and for a penalty in an amount determined at the rate of 12 percent per year upon the amount of any underpayment of estimated tax determined under this subsection.

(b) For purposes of this subsection, the amount of any underpayment of estimated tax shall be the excess of:

1. The amount of the installment which would be required to be paid if the estimated tax were equal to 90 percent of the tax shown on the return for the taxable year or, if no return were filed, 90 percent of the tax for such year, over

2. The amount, if any, of the installment paid on or before the last date prescribed for payment.

(c) The period of the underpayment for which interest and penalties apply shall commence on the date the installment was required to be paid, determined without regard to any extensions of time, and shall terminate on the earlier of the following dates:

1. The 1st day of the 5th month after the close of the taxable year;

2. For taxable years beginning before January 1, 2026, for taxpayers with a taxable year ending June 30, the 1st day of the 4th month after the close of the taxable year; or

3. With respect to any portion of the underpayment, the date on which such portion is paid.

For purposes of this paragraph, a payment of estimated tax on any installment date shall be considered a payment of any previous underpayment only to the extent such payment exceeds the amount of the installment determined under subparagraph (b)1. for such installment date.

(d) No penalty or interest for underpayment of any installment of estimated tax may be imposed if the total amount of all such payments made on or before the last date prescribed for the payment of such installment equals or exceeds the amount which would have been required to be paid on or before such date if the estimated tax were the lesser amount of:

1. An amount equal to the tax computed at the rates applicable to the taxable year, but otherwise on the basis of the facts shown on the return for, and the law applicable to, the preceding taxable year; or

2. An amount equal to 90 percent of the tax finally due for the taxable year.

(e) For purposes of paragraphs (b) and (d), the term “tax” means the excess of the tax imposed by this code over all amounts properly credited against such tax for the taxable year.

(f) The application of this subsection to taxable years of less than 12 months shall be in accordance with regulations prescribed by the department.

(3) The department may provide by regulation for a credit against estimated taxes for any taxable year of any amount determined by the taxpayer or by the department to be an overpayment of the tax imposed by this code for a preceding taxable year.