; and (B) the head of the tribal council. (2) The term tribal council means the governing body of a federally recognized Indian tribe. (c) Effective date of agreement Any agreement or modification of an agreement under this section shall be effective with respect to services performed after an effective date specified in such agreement or modification, except that such date may not be earlier than the first day of the next calendar month after which the Indian tribe signs the agreement and sends it to the Commissioner for signature. (d) Duration of agreement No agreement under this section may be terminated, either in its entirety or with respect to any member, on or after the effective date of the agreement. . (b) Conforming amendment Section 3121(d)(4) of the Internal Revenue Code of 1986 is amended by inserting or 218A after section 218 . (c) Rule of construction Nothing in this Act or the amendments made by this Act shall be construed to affect application of any Federal income tax withholding requirements under the Internal Revenue Code of 1986.