26 C.F.R. § 1.6013-3

For purposes of section 21 (relating to change in rates during a taxable year), section 443 (relating to returns for a period of less than 12 months), and section 7851(a)(1)(A) (relating to the applicability of certain provisions of the Internal Revenue Code of 1954 and the Internal Revenue Code of 1939), where the husband and wife have different taxable years because of death of either spouse, the joint return shall be treated as if the taxable years of both ended on the date of the closing of the surviving spouse's taxable year. Thus, in cases where the Internal Revenue Code of 1939 otherwise would apply to the taxable year of the decedent spouse and the Internal Revenue Code of 1954 would apply to the taxable year of the surviving spouse, this provision makes the Internal Revenue Code of 1954 applicable to the taxable years of both spouses if a joint return is filed.