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SI Units - Time

s - Second - Time - 2018

The second (s) is defined by taking the fixed numerical value of the cesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s−1.

The number of periods or cycles per second is called frequency. The SI unit for frequency is the hertz (Hz). One hertz is the same as one cycle per second. Standard frequencies and the correct time are broadcast by radio stations WWV and WWVB in Colorado, and WWVH in Hawaii. NIST delivers digital timing signals by telephone and through the internet.

Official U.S. Government time is provided by NIST and USNO. NIST also offers an Internet Time Service (ITS) and an Automated Computer Time Service (ACTS) that allow setting of computer and other clocks through the Internet or over standard commercial telephone lines. Free software for using these services on several types of popular computers can be downloaded there. Information about these services can be found on the Time and Frequency Division Web site.


League of SI Superheroes – Professor Second

This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. By reading the vibrations of her laser-cooled cesium atoms, Professor Second can synchronize any frequency and correct any clock. A second is the time it takes an excited cesium atom to vibrate 9,192,631,770 times.

Measuring Time and Frequency

Find out more about NIST's history including a brief history of atomic clocks and other time Frequently Asked Questions (FAQs).

Take A Walk Through Time

Follow the historic journey through of the evolution of time measurement. PDF (Click link to download the attached file)

The Time - Place Connection

If you want to know where you are, you need a reliable clock. Dive into these Smithsonian resources to discover more about the connection between time and place.

It's About Time

Check out the University of Wisconsin's exhibit. Learn cool things about the accuracy of clocks, including a historical Timeline (pun intended)!


What is the current Coordinated Universal Time?

NIST and the U.S. Naval Observatory jointly operate a website that provides the Official U.S. Time. Readings from the clocks of these two agencies contribute to world time, called Coordinated Universal Time (UTC). Learn more...How to get the time using a telephone, computer or radio signals? What is a leap second? What are the daylight saving time rules? Visit the Time and Frequency Division FAQs for more information.


Created April 12, 2010, Updated November 15, 2019