was made for purposes of clause (i), a payment of estimated tax shall be credited against unpaid required installments in the order in which such installments are required to be paid. . (b) Effective Date The amendments made by this section shall apply to taxable years beginning after December 31, 2017. 305. Modifications to computation of individual estimated tax (a) In general Section 6654 is amended by striking subsections (a) and (b) and inserting the following: (a) Addition to the Tax (1) In general Except as otherwise provided in this section, in the case of any underpayment of estimated tax by an individual for a taxable year, there shall be added to the tax under chapter 1, the tax under chapter 2, and the tax under chapter 2A for such taxable year the sum of the amounts determined under paragraph (2) for each day for which there is an underpayment. (2) Amount of addition to tax The amount determined under this paragraph for any day shall be the product of— (A) the underpayment rate established under subsection (b)(2) for such day, multiplied by (B) the amount of the underpayment for such day. (b) Amount of underpayment