Section 71.84 - Addition to the tax(1) INDIVIDUALS AND FIDUCIARIES. Except as provided in s. 71.09 (11), in the case of any underpayment of estimated tax by an individual, estate or trust, except as provided under s. 71.09, there shall be added to the aggregate tax for the taxable year interest at the rate of 12 percent per year on the amount of the underpayment for the period of the underpayment. In this subsection, "the period of the underpayment" means the time period from the due date of the installment until either the 15th day of the 4th month beginning after the end of the taxable year or the date of payment, whichever is earlier.(2) CORPORATIONS. (a) Except as provided in s. 71.29 (7), in the case of any underpayment of estimated tax by a corporation under s. 71.29 or 71.48, there shall be added to the aggregate tax for the taxable year interest at the rate of 12 percent per year on the amount of the underpayment for the period of the underpayment. In this paragraph, "period of the underpayment" means the time period from the due date of the installment until either the date on which the corporation is required to file for federal income tax purposes, not including any extension, under the Internal Revenue Code or the date of payment, whichever is earlier. If 90 percent of the tax shown on the return is not paid by the date on which the corporation is required to file for federal income tax purposes, not including any extension, under the Internal Revenue Code, the difference between that amount and the estimated taxes paid, along with any interest due, shall accrue delinquent interest under s. 71.91 (1) (a).(b) For corporations that are subject to a tax under this chapter on unrelated business taxable income, as defined under section 512 of the internal revenue code, and virtually exempt entities, "period of the underpayment" means the time period from the due date of the installment until either the 15th day of the 5th month beginning after the end of the taxable year or the date of payment, whichever is earlier. If 90 percent of the tax shown on the return is not paid by the 15th day of the 5th month following the close of the taxable year, the difference between that amount and the estimated taxes paid along with any interest due, shall accrue delinquent interest under s. 71.91 (1) (a).(c) If a refund under s. 71.29 (3m) results in an income or franchise tax liability that is greater than the amount of estimated taxes paid when reduced by the amount of the refund, the taxpayer shall add to the aggregate tax for the taxable year interest at an annual rate of 12 percent on the amount of the unpaid tax liability for the period beginning on the date the refund is issued and ending on either the date on which the taxpayer is required to file for federal income tax purposes, not including any extension, under the Internal Revenue Code or the date the tax liability is paid, whichever is earlier.Amended by Acts 2017 ch, 2,s 12, eff. 3/11/2017.Amended by Acts 2017 ch, 2,s 11, eff. 3/11/2017.1987 a. 312, 411; 1989 a. 31; 1991 a. 39. The definition of "return" in s. 71.29 applies to both the first and last sentences in sub. (2) (a). Thus, "return" means a return showing the proper amount due, regardless of whether the audited amount due is higher than the amount shown on the filed return. General Casualty Company v. DOR, 2002 WI App 248, 258 Wis. 2d 196, 653 N.W.2d 513, 01-2810.