The National Institute of Standards and Technology and the United States Naval Observatory are both responsible for keeping the official U.S. time. The two agencies coordinate their efforts to ensure that they are as synchronized as possible.
Both agencies use a combination of cesium atomic clocks and hydrogen maser clocks to ensure the continued accuracy of their timekeeping. A comparison of the readouts from these clocks takes place every 100 seconds. This keeps the official time accurate to within 100 picoseconds per day. Both organizations contribute to Coordinated Universal Time, the international time standard maintained by the International Bureau of Weights and Measures in France.