(ii) Deferment (I) In general In the case of any taxpayer described in subclause (II) for the taxable year, no tax shall be imposed under subparagraph (A) for such taxable year and the number of taxable years in the offset period shall be increased (other than for purposes of subparagraph (B)(ii)) by 1 year. (II) Taxpayer described A taxpayer is described in this subclause for any taxable year if the earned income amount of the taxpayer for such taxable year is more than 20 percent less than the earned income amount of the taxpayer for the preceding taxable year and the taxpayer makes an election under this clause. The Secretary shall promulgate rules for calculating differences in the earned income amount of any taxpayer whose filing status is not the same as such taxpayer's filing status in the preceding taxable year. (III) Election An election made under this clause shall apply to all applicable qualifying children of the taxpayer with respect to whom an increase is allowed under this subsection and may not be made with respect to more than 3 taxable years in any offset period. (D) Joint returns In the case of an increase in credit allowed under paragraph (1) with respect to a joint return, half of such increase shall be treated as having been allowed to each individual filing such return for purposes of this paragraph. (E) Return requirement If the tax imposed by this chapter for the taxable year is increased under this paragraph, the taxpayer shall, notwithstanding section 6012, be required to file a return with respect to the taxes imposed under this subtitle. (F) Offset period (i) In general Except as provided in subparagraph (C)(ii), the term offset period means, with respect to any increase under this paragraph, the 10 taxable years beginning with the taxable year in which the applicable qualifying child is born or adopted. (ii) Special rule In the case of a qualifying child who is adopted by the taxpayer and who is over the age of 5 on the date the adoption becomes final, the number of taxable years in