When the provisions of sections 14 or 15 apply, the permissible period for the postponement of a vesting of a future interest shall be fixed in accordance with either the law in effect at the time of the exercise of the power or of the termination of the unqualified power of amendment or revocation, or in accordance with the law in effect at the time of the creation of the power, whichever will support the validity of the exercise of the power.