26 CFR § 54.4971-1 - General rules relating to excise tax on failure to meet minimum funding standards.
(c) Additional tax. Section 4971(b) imposes an excise tax in any case in which an initial tax is imposed under section 4971(a) on an accumulated funding deficiency and the accumulated funding deficiency is not corrected within the taxable period (as defined in section 4971(c)(3)). The additional tax is 100 percent of the accumulated funding deficiency to the extent not corrected.
(e) Definition of taxable period -
(1) In general. For purposes of any accumulated funding deficiency, the term “taxable period” means the period beginning with the end of the plan year in which there is an accumulated funding deficiency and ending on the earlier of:
(i) The date of mailing of a notice of deficiency under section 6212 with respect to the tax imposed by section 4971(a), or
(ii) The date on which the tax imposed by section 4971(a) is assessed.
(2) Special rule. Where a notice of deficiency referred to in paragraph (e)(1)(i) of this section is not mailed because a waiver of the restrictions on assessment and collection of a deficiency has been accepted or because the deficiency is paid, the date of filing of the waiver or the date of such payment, respectively, shall be treated as the end of the taxable period.