28 U.S.C. App. (FEDERAL RULES OF APPELLATE PROCEDURE) Rule 37

(a) When the Court Affirms. Unless the law provides otherwise, if a money judgment in a civil case is affirmed, whatever interest is allowed by law is payable from the date when the district court’s judgment was entered. (b) When the Court Reverses. If the court modifies or reverses a judgment with a direction that a money judgment be entered in the district court, the mandate must contain instructions about the allowance of interest.