Reimbursement of funds advanced by non-Federal purchasers for the uprating program shall be a repayment requirement of the Boulder Canyon project beginning with the first day of the month following completion of each segment thereof. The cost of the visitor facilities program as defined in section 619(a) of this title shall become a repayment requirement beginning June 1, 1987, or when substantially completed, as determined by the Secretary of the Interior, if later.