What is a national time clock?

The national time clock for the United states is the clock that is used to measure the time used as the country's standard time. It is known as the Master Clock and is maintained by the United States Naval Observatory, which continuously displays the current time on its website.

The USNO Master Clock is not one clock, but a collection of clocks used to measure precise time. As of March 2015, the clocks used for the Master Clock include 12 hydrogen maser clocks and a system of cesium atomic clocks. These clocks are kept in 20 different controlled vaults and are compared every 100 seconds to produce a stable, reliable time.