Today a “leap second” will be added to the world’s clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC), according to the U.S. Naval Observatory. This corresponds to 6:59:59 pm Eastern Standard Time, when the extra second will be inserted at the U.S. Naval Observatory’s Master Clock Facility in Washington, DC.
According to a press release, this marks the 24 th leap second to be added to UTC, a uniform time-scale kept by atomic clocks around the world. In fact, since 1972, leap seconds have been added at intervals varying from six months to seven years, with the last being inserted on December 31, 2005.
The U.S. Naval Observatory notes that historically time was based on the mean rotation of the earth relative to celestial bodies and the second was defined in this reference frame. “However, the invention of atomic clocks defined a much more precise “atomic time” scale and a second that is independent of the earth’s rotation.”
In 1970, an international agreement established two timescales: one based on the rotation of the earth and one based on atomic time. According to an announcement, the problem is that the earth’s rotation is very gradually slowing down, which necessitates the periodic insertion of a “leap second” into the atomic timescale to keep the two within 1 second of each other. The International Earth Rotation and Reference Systems Service (IERS) is the organization which monitors the difference in the two timescales and calls for leap seconds to be inserted or removed when necessary.
You can check the correct time at http://tycho.usno.navy.mil/what.html