26 U.S.C. § 645

(a) General rule For purposes of this subtitle, if both the executor (if any) of an estate and the trustee of a qualified revocable trust elect the treatment provided in this section, such trust shall be treated and taxed as part of such estate (and not as a separate trust) for all taxable years of the estate ending after the date of the decedent’s death and before the applicable date. (b) Definitions For purposes of subsection (a)— (1) Qualified revocable trust The term “qualified revocable trust” means any trust (or portion thereof) which was treated under section 676 as owned by the decedent of the estate referred to in subsection (a) by reason of a power in the grantor (determined without regard to section 672(e)). (2) Applicable date The term “applicable date” means— (A) if no return of tax imposed by chapter 11 is required to be filed, the date which is 2 years after the date of the decedent’s death, and (B) if such a return is required to be filed, the date which is 6 months after the date of the final determination of the liability for tax imposed by chapter 11. (c) Election The election under subsection (a) shall be made not later than the time prescribed for filing the return of tax imposed by this chapter for the first taxable year of the estate (determined with regard to extensions) and, once made, shall be irrevocable.