However, a taxpayer may choose to apply the rules of this section for a taxable year beginning after December 31, 2017, the effective date of the revision of section 807 by Public Law 115-97 , and on or before October 13, 2020, provided the taxpayer consistently applies the rules of this section to that taxable year and all subsequent taxable years. See section 7805(b)(7).
26 C.F.R. §1.816-1