((a)) ** .—** A judgment against the United States or a federally-owned corporation under this chapter may include costs and interest at the rate of 4 percent per year until satisfied. Interest shall run as ordered by the court, except that interest is not allowable for the period before the action is filed.

((b)) ** .—** Notwithstanding subsection (a), if the claim is based on a contract providing for interest, interest may be awarded at the rate and for the period provided in the contract.