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Another Leap Second is at Hand

Though your summer this year may seem all too short, it will actually be the longest one since 1994. There will be exactly one extra second, a little bonus added to allow the world’s atomic clocks to be synchronized to the spinning Earth.

The U.S. timekeepers, the National Institute of Standards and Technology’s Time and Frequency Division in Boulder, Colo., and the U.S. Naval Observatory in Washington, D.C., will be adding a leap second on June 30, along with the operators of standard clocks in all of the world’s scientifically developed countries. This is the 21st adjustment of this kind to the world’s time scale (as decreed by the International Earth Rotation Service in France) since 1972.

Leap seconds are needed to keep our clocks (the best of which, like those at NIST and USNO, are now so accurate that they lose or gain much less than a millionth of a second in a year) in step with the spinning Earth, which varies several thousandths of a second per day. Since we cannot speed up the Earth, we have to slow down the clocks to keep them “in sync.” In effect, this is done by stopping the clocks for exactly one second every year or two to allow the Earth to catch up.

This year’s leap second will be inserted at 23:59:60 UTC on June 30, 1997. That’s the technical way to say it will happen just before 8 p.m. EDT (7 p.m. CDT, 6 p.m. MDT and 5 p.m. PDT). It will make the last day of June one second longer than normal.

The last leap second was on December 31, 1995, and the last summertime leap second was on June 30, 1994. The first leap seconds were in June and December of 1972, the only year that ever had two. All of the others were inserted at intervals of 12 to 30 months apart.

For most people, a single second more or less doesn’t matter much. But for some, a second is a big deal. Modern television, telephone and other telecommunication systems; marine and aviation navigation systems; computer networks; electric power grids; and multitudes of scientific applications are among the critical activities that depend on very precise time and frequency. Many of these will have to be adjusted for the leap second to maintain synchronism with the rest of the world.

As a non-regulatory agency of the Commerce Department’s Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.

The NIST Boulder Laboratories website is at,

Released May 27, 1997, Updated November 27, 2017