How Does Atomic Time Get to Your Computer?

NIST staff in Boulder, Colorado operate an ensemble of commercial atomic clocks to produce official U.S. time.

The signals from those clocks are averaged to produce a single wave that oscillates 5 million times per second.

Every 5 millionth cycle of that wave, a green light flashes. Each flash marks the beginning of an official second in the United States.

NIST staff also run custom-built cesium fountain clocks. These ultra-precise clocks don't keep time; instead, they are like tuning forks that tune the other clocks in NIST's ensemble.

NIST sends data from its clocks to the International Bureau of Weights and Measures (BIPM) in France. Around 90 other clock labs also send in data.

BIPM produces a weighted average of all the data to create the time scale known as Coordinated Universal Time, or UTC.

(A time scale is an agreed-upon system for keeping time using data from clocks around the world.)

BIPM tells every contributing clock lab how far off its clocks are from UTC.

NIST timekeepers adjust their own time scale, known as UTC(NIST).

UTC(NIST) is broadcast via radio stations based in Fort Collins, Colorado, and the island of Kauai in Hawaii, and distributed to the internet via the Internet Time Service, which receives 1 million hits per second — more than Google.

Your computer receives the time from an internet hub and adjusts its internal clock.