Sorry, you need to enable JavaScript to visit this website.
Skip to main content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.

Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.


The site is secure.

The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Time to End the Leap Second?

November 6, 2015

We all rely on clocks to figure out when to leave for work, go to school and to do a myriad of other activities that make up our daily lives. But how we keep track of time on those clocks is a subject of debate among the nations of the world. Most countries use the international standard time scale called Coordinated Universal Time (UTC). The U.S. Commerce Department’s National Institute of Standards and Technology (NIST), NTIA’s sister agency, maintains the UTC time scale in the United States.

UTC is based on atomic clocks, which use the international definition of the “second” to keep highly accurate and very steady track of time.  Atomic clocks have been used for time-keeping services since the 1950s. Traditional clocks are based on astronomical measures of the Earth’s rotation. The Earth’s rotation rate is slowing down over time, however, and the rate of that slowdown is irregular. To keep UTC in line with the Earth’s rotation, a second is added to UTC time when needed to ensure that the difference between traditional clocks and atomic clocks is less than a second. One of these seconds, a so-called “leap second,” was most recently added at the end of June.  Since the leap second’s introduction in 1972, 26 leap seconds have been added to UTC to keep the world’s clocks in sync with the Earth’s rotation.

The United States, joined by its peers in the Inter-American Telecommunication Commission (CITEL), have developed a proposal to cease the use of leap seconds in UTC. This issue will be taken up later this month in Geneva where more than 190 nations are meeting for the International Telecommunication Union’s (ITU) World Radio Communications Conference (WRC-15). While the Department of State is heading the U.S. delegation to the WRC-15, NTIA has the lead on the leap second issue at the conference.

Because the world’s time keepers can’t predict when they will need to add a leap second, computer systems and telecommunications networks around the world must be adjusted manually to account for the extra second added to UTC. These occasional disruptions to UTC have caused some glitches over the years. Perhaps the most famous of these occurred in 2012 when the addition of the leap second disrupted the Qantas Airlines’ flight reservation system, resulting in that system going offline and delaying hundreds of flights. The most recent addition of the leap second at the end of June caused some Internet outages that were quickly resolved.

Defenders of the leap second voice concern about the historical precedent of moving away from linking time to the Earth’s rotation. They note that eventually the sunrise and sunset would be out of sync with what our clocks say.  However, even a difference of a minute would take many decades to accumulate.

In addition to ending use of the leap second, the United States also supports shifting responsibility for the UTC time scale from the ITU to the International Bureau of Weights and Measures (BIPM), the international organization in charge of time standards and units of measure. ITU’s role related to UTC stems from its responsibility in distributing standard frequency and time signal via radio communication.  But the United States believes that the BIPM is better equipped to maintain UTC than an international organization dedicated to telecommunications.

The United States is hopeful the ITU’s member states can come to consensus to resolve this important issue. By establishing a constant time reference for the world, we will minimize future disruptions to the vast digital infrastructure on which our modern world relies.

In addition to the leap second issue, there are more than 30 agenda items being addressed at WRC-15. Two of the top U.S. priorities for WRC-15 are identifying additional spectrum for International Mobile Telecommunications (IMT) or more generally called mobile broadband, and the identification of spectrum to support beyond-line-of-sight command and control of Unmanned Aircraft Systems (UAS).  Both of these items can significantly contribute to the economic growth and well-being of the United States as well as many other developed and developing countries.  The United States also joined a CITEL proposal for a future conference agenda item to identify additional spectrum (above 6 GHz) for the fifth generation of wireless networks known as 5G,  which will be a critical step toward achieving 5G capabilities within the United States and globally.