; underpayment rate For purposes of subsection (a)— (1) Amount The amount of underpayment on any day shall be the excess (if any) of— (A) the sum of the required installments for the taxable year the due dates for which are on or before such day, over (B) the sum of the amounts of estimated tax payments made for such taxable year on or before such day. (2) Determination of underpayment rate (A) In general The underpayment rate with respect to any day in an installment underpayment period shall be the underpayment rate established under section 6621 for the first day of the calendar quarter in which such installment underpayment period begins. (B) Installment underpayment period For purposes of subparagraph (A), the term installment underpayment period means the period beginning on the day after the due date for a required installment and ending on the due date for the subsequent required installment (or in the case of the 4th required installment, the 15th day of the 4th month following the close of a taxable year). (C) Daily rate The rate determined under subparagraph (A) shall be applied on a daily basis and shall be based on the assumption of 365 days in a calendar year. (3) Termination of estimated tax underpayment No day after the end of the installment underpayment period for the 4th required installment specified in paragraph (2)(B) for a taxable year shall be treated as a day of underpayment with respect to such taxable year. . (b) Effective Date The amendments made by this section shall apply to taxable years beginning after December 31, 2017. 306. Corporate estimated tax (a) Increase in Small Tax Amount Exception Section 6655(f) is amended by striking $500 and inserting $2,000 . (b) Effective Date The amendment made by this section shall apply to taxable years beginning after December 31, 2017. 307. Increase in large corporation threshold for estimated tax payments (a) In General Section 6655(g)(2) is amended— (1) by striking $1,000,000 in subparagraph (A) and inserting $1,500,000 , (2) by striking the