Cesium beam clocks (left) and hydrogen masers are among the types of atomic clocks used by the National Institute of Standards and Technology (NIST) to determine official U.S. time. J. Sherman, R. Jacobson/National Institute of Standards and Technology hide caption toggle caption J. Sherman, R. Jacobson/National Institute of Standards and Technology The U.S. government calculates the country's official time using more than a dozen atomic clocks at a federal facility northwest of Denver. But wh...