Section 672(d) provides that a person is considered to have a power described in subpart E (section 671 and following), part I, subchapter J, chapter 1 of the Code, even though the exercise of the power is subject to a precedent giving of notice or takes effect only after the expiration of a certain period of time. However, although a person may be considered to have such a power, the grantor will nevertheless not be treated as an owner by reason of the power if its exercise can only affect beneficial enjoyment of income received after the expiration of a period of time such that, if the power were a reversionary interest, he would not be treated as an owner under section 673. See sections 674(b)(2), 676(b), and the last sentence of section 677(a). Thus, for example, if a grantor creates a trust for the benefit of his son and retains a power to revoke which takes effect only after the expiration of 2 years from the date of exercise, he is treated as an owner from the inception of the trust. However, if the grantor retains a power to revoke, exercisable at any time, which can only affect the beneficial enjoyment of the ordinary income of a trust received after the expiration of 10 years commencing with the date of the transfer in trust, or after the death of the income beneficiary, the power does not cause him to be treated as an owner with respect to ordinary income during the first 10 years of the trust or during the income beneficiary's life, as the case may be. See section 676(b).