; and (2) by adding at the end the following new paragraph: (4) Individual retirement plans For purposes of any tax imposed by section 4973, 4974, or 4975 in connection with an individual retirement plan, the return referred to in this section shall be the income tax return filed by the person on whom the tax under such section is imposed for the year in which the act (or failure to act) giving rise to the liability for such tax occurred. In the case of a person who is not required to file an income tax return for such year— (A) the return referred to in this section shall be the income tax return that such person would have been required to file but for the fact that such person was not required to file such return, and (B) the 3-year period referred to in subsection (a) with respect to the return shall be deemed to begin on the date by which the return would have been required to be filed (excluding any extension thereof). . (f) Effective date (1) In general Subject to paragraphs (2) and (3), this section and the amendments made by this section shall take effect on the date of the enactment of this Act. (2) Transition provisions (A) In general The amendments made by this section shall apply to any determination of or affecting liability for taxes, interest, or penalties which is made on or after the date of the enactment of this Act, without regard to whether the conduct upon which the determination is based occurred before such date of enactment. (B) Calculation of correction window in certain cases In the case of an error that would have been eligible for correction under section 4973(i) or 4974(e) of the Internal Revenue Code of 1986 if tax had not been imposed under 4973(a) or 4974(a), as the case may be, of such Code before the date of the enactment of this Act, the correction window referred to in sections 4973(i) and 4974(e) of such Code (as added by this section) shall be the period beginning on the date on which such tax was imposed and ending on the earlier of— (i) the date on which the Secretary of