26 C.F.R. § 1.673(d)-1
Any postponement of the date specified for the reacquisition of possession or enjoyment of any reversionary interest is considered a new transfer in trust commencing with the date on which the postponement is effected and terminating with the date prescribed by the postponement. However, the grantor will not be treated as the owner of any portion of a trust for any taxable year by reason of the foregoing sentence if he would not be so treated in the absence of any postponement. The rules contained in this section may be illustrated by the following example:
Example. G places property in trust for the benefit of his son B. Upon the expiration of 12 years or the earlier death of B the property is to be paid over to G or his estate. After the expiration of 9 years G extends the term of the trust for an additional 2 years. G is considered to have made a new transfer in trust for a term of 5 years (the remaining 3 years of the original transfer plus the 2-year extension). However, he is not treated as the owner of the trust under section 673 for the first 3 years of the new term because he would not be so treated if the term of the trust had not been extended. G is treated as the owner of the trust, however, for the remaining 2 years.
26 C.F.R. §1.673(d)-1