26 U.S.C. § 6075

(a) Estate tax returns Returns made under section 6018(a) (relating to estate taxes) shall be filed within 9 months after the date of the decedent’s death. (b) Gift tax returns (1) General rule Returns made under section 6019 (relating to gift taxes) shall be filed on or before the 15th day of April following the close of the calendar year. (2) Extension where taxpayer granted extension for filing income tax return Any extension of time granted the taxpayer for filing the return of income taxes imposed by subtitle A for any taxable year which is a calendar year shall be deemed to be also an extension of time granted the taxpayer for filing the return under section 6019 for such calendar year. (3) Coordination with due date for estate tax return Notwithstanding paragraphs (1) and (2), the time for filing the return made under section 6019 for the calendar year which includes the date of death of the donor shall not be later than the time (including extensions) for filing the return made under section 6018 (relating to estate tax returns) with respect to such donor.