due date
(3) Computation of interest (A) In general The amount of interest referred to in paragraph (1)(B) on any increase determined under paragraph (2) for any taxable year shall be determined for the period— (i) beginning on the due date for such taxable year, and (ii) ending on the due date for the taxable year with or within which the distribution or disposition occurs, by using the rates and method applicable under section 6621 for underpayments of tax for such period. (B) Due date For purposes of this subsection, the term “due date” means the date prescribed by law (determined without regard to extensions) for filing the return of the tax imposed by this chapter for the taxable year.