(a) The income of an estate of a deceased person is that which is received by the estate during the period of administration or settlement. The period of administration or settlement is the period actually required by the administrator or executor to perform the ordinary duties of administration, such as the collection of assets and the payment of debts, taxes, legacies, and bequests, whether the period required is longer or shorter than the period specified under the applicable local law for the settlement of estates. For example, where an executor who is also named as trustee under a will fails to obtain his discharge as executor, the period of administration continues only until the duties of administration are complete and he actually assumes his duties as trustee, whether or not pursuant to a court order. However, the period of administration of an estate cannot be unduly prolonged. If the administration of an estate is unreasonably prolonged, the estate is considered terminated for Federal income tax purposes after the expiration of a reasonable period for the performance by the executor of all the duties of administration. Further, an estate will be considered as terminated when all the assets have been distributed except for a reasonable amount which is set aside in good faith for the payment of unascertained or contingent liabilities and expenses (not including a claim by a beneficiary in the capacity of beneficiary). Notwithstanding the above, if the estate has joined in making a valid election under section 645 to treat a qualified revocable trust, as defined under section 645(b)(1), as part of the estate, the estate shall not terminate under this paragraph prior to the termination of the section 645 election period. See section 645 and the regulations thereunder for rules regarding the termination of the section 645 election period.

(b) Generally, the determination of whether a trust has terminated depends upon whether the property held in trust has been distributed to the persons entitled to succeed to the property upon termination of the trust rather than upon the technicality of whether or not the trustee has rendered his final accounting. A trust does not automatically terminate upon the happening of the event by which the duration of the trust is measured. A reasonable time is permitted after such event for the trustee to perform the duties necessary to complete the administration of the trust. Thus, if under the terms of the governing instrument, the trust is to terminate upon the death of the life beneficiary and the corpus is to be distributed to the remainderman, the trust continues after the death of the life beneficiary for a period reasonably necessary to a proper winding up of the affairs of the trust. However, the winding up of a trust cannot be unduly postponed and if the distribution of the trust corpus is unreasonably delayed, the trust is considered terminated for Federal income tax purposes after the expiration of a reasonable period for the trustee to complete the administration of the trust. Further, a trust will be considered as terminated when all the assets have been distributed except for a reasonable amount which is set aside in good faith for the payment of unascertained or contingent liabilities and expenses (not including a claim by a beneficiary in the capacity of beneficiary).

(c)(1) Except as provided in subparagraph (2) of this paragraph, during the period between the occurrence of an event which causes a trust to terminate and the time when the trust is considered as terminated under this section, whether or not the income and the excess of capital gains over capital losses of the trust are to be considered as amounts required to be distributed currently to the ultimate distributee for the year in which they are received depends upon the principles stated in § 1.651(a)-2. See § 1.663-1 et seq. for application of the separate share rule.

(2)(i) Except in cases to which the last sentence of this subdivision applies, for taxable years of a trust ending before September 1, 1957, subparagraph (1) of this paragraph shall not apply and the rule of subdivision (ii) of this subparagraph shall apply unless the trustee elects to have subparagraph (1) of this paragraph apply. Such election shall be made by the trustee in a statement filed on or before April 15, 1959, with the district director with whom such trust's return for any such taxable year was filed. The election provided by this subdivision shall not be available if the treatment given the income and the excess of capital gains over capital losses for taxable years for which returns have been filed was consistent with the provisions of subparagraph (1) of this paragraph.

(ii) The rule referred to in subdivision (i) of this subparagraph is as follows: During the period between the occurrence of an event which causes a trust to terminate and the time when a trust is considered as terminated under this section, the income and the excess of capital gains over capital losses of the trust are in general considered as amounts required to be distributed for the year in which they are received. For example, a trust instrument provides for the payment of income to A during her life, and upon her death for the payment of the corpus to B. The trust reports on the basis of the calendar year. A dies on November 1, 1955, but no distribution is made to B until January 15, 1956. The income of the trust and the excess of capital gains over capital losses for the entire year 1955, to the extent not paid, credited, or required to be distributed to A or A's estate, are treated under sections 661 and 662 as amounts required to be distributed to B for the year 1955.

(d) If a trust or the administration or settlement of an estate is considered terminated under this section for Federal income tax purposes (as for instance, because administration has been unduly prolonged), the gross income, deductions, and credits of the estate or trust are, subsequent to the termination, considered the gross income, deductions, and credits of the person or persons succeeding to the property of the estate or trust.