by adding at the end the following new paragraph: (4) Exception for start-up losses (A) In general In the case of any net operating loss carryforward described in paragraph (1)(A) which arose in a start-up period taxable year, the amount of such net operating loss carryforward otherwise taken into account under such paragraph shall be reduced by the net start-up loss determined with respect to the trade or business referred to in subparagraph (B)(i) for such start-up period taxable year. (B) Start-up period taxable year The term start-up period taxable year means any taxable year of the old loss corporation which— (i) begins before the close of the 3-year period beginning on the date on which any trade or business of such corporation begins as an active trade or business (as determined under section 195(d)(2) without regard to subparagraph (B) thereof), and (ii) ends after September 10, 2018. (C) Net start-up loss (i) In general The term net start-up loss means, with respect to any trade or business referred to in subparagraph (B)(i) for any start-up period taxable year, the amount which bears the same ratio (but not greater than one) to the net operating loss carryforward which arose in such start-up period taxable year as— (I) the net operating loss (if any) which would have been determined for such start-up period taxable year if only items of income, gain, deduction, and loss properly allocable to such trade or business were taken into account, bears to (II) the amount of the net operating loss determined for such start-up period taxable year. (ii) Special rule for last taxable year in start-up period In the case of any start-up period taxable year which ends after the close of the 3-year period described in subparagraph (B)(i) with respect to any trade or business, the net start-up loss with respect to such trade or business for such start-up period taxable year shall be the same proportion of such loss (determined without regard to this clause) as the proportion of such start-up period taxable year which