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.