For purposes of sections
3721 and
3792 of this title, interest shall be computed, at a rate determined by the Secretary of the Treasury, on the investment of the United States in the Panama Canal as shown in the accounts of the Panama Canal Company at the close of business on the day preceding October 1, 1979, and as adjusted in accordance with subsections (b) and (c) of this section. Capital investment for interest purposes shall not include any interest during construction.