The Federal Prompt Payment Act

The Prompt Payment Final Rule (formerly OMB Circular A-125, "Prompt Payment") requires Executive departments and agencies of the Federal Government to pay commercial obligations within certain time periods and to pay interest penalties when payments are late.

With certain exceptions, the Act requires that the Government make payment within 30 days from the date of submission of a properly prepared invoice by a contractor. For amounts not paid within the required period, the Government is obligated to pay interest at a rate established by the Secretary of the Treasury. At the time of original enactment, the law provided for a 15-day grace period in addition to the basic 30-day period; this provision was subsequently repealed due to perceived abuse by the Government.

http://www.fms.treas.gov/prompt/5cfr1315.pdf