From the USA.gov Blog:
A Leap Second Will Be Added to Clocks:
On Saturday, the Naval Observatory will add a “leap second” to the world’s clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). This corresponds to 7:59:59 pm Eastern Daylight Time.
Time, including seconds, has historically been defined by the rotation of the Earth around other objects in space. The invention of atomic clocks allowed us to more precisely define time.
In 1970, an international agreement established two timescales: one based on the rotation of the Earth and one on the atomic clock. These timescales are not exactly the same so extra seconds are added or removed to keep them within 0.9 seconds of each other. Saturday’s “leap second” will help sync the two timescales.
The Naval Observatory and National Institute of Standards and Technology are responsible for determining the time in the United States.
Learn more about the addition of this leap second to the clock. (PDF)