Like the more well-known time adjustment, the leap year, a "leap second" is tacked on to clocks every so often to keep them correct.
Earth's trip around the sun — our year with all its seasons — is about 365.2422 days long, which we round to 365 to keep things simpler. But every four years, we add 0.2422 x 4 days (that's about one day) at the end of the month of February (extending it from 28 to 29 days) to fix the calendar.
Likewise, a "leap second" is added on to our clocks every so often to keep them in synch with the somewhat unpredictable nature of our planet's rotation, the roughly 24-hour whirl that brings the sun into the sky each morning.
Historically, time was based on the mean rotation of the Earth relative to celestial bodies and the second was defined from this frame of reference. But the invention of atomic clocks brought about a definition of a second that is independent of the Earth's rotation and based on a regular signal emitted by electrons changing energy state within an atom.
In 1970, an international agreement established two timescales: one based on the rotation of the Earth and one based on atomic time.
The problem is that the Earth is very gradually slowing down, continually throwing the two timescales out of synch, so every so often, a "leap second" has to be tacked on to the atomic clock.
The International Earth Rotation and Reference Systems Service is the organization that monitors the difference in the two timescales and calls for leap seconds to be inserted or removed when necessary. Since 1972, leap seconds have been added at intervals varying from six months to seven years — the most recent was inserted on Dec. 31, 2005.
In the United States, the U.S. Naval Observatory and the National Institute of Standards and Technology keep time for the country. The Naval Observatory keeps the Department of Defense's Master Clock, an atomic clock located in Washington, D.C.
The new extra second will be added on the last day of this year at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time — 6:59:59 pm Eastern Standard Time.
Mechanisms such as the Internet-based Network Time Protocol and the satellite-based Global Positioning System (GPS) depend on the accurate time kept by atomic clocks.