This year to be one second longer: US Naval Observatory

WASHINGTON: On December 31 this year, a "leap second" will be added to the world's clocks at 23 hours, 59 minutes 59 seconds Coordinated Universal Time, the US Naval Observatory has announced.
This corresponds to 4:29:59 am IST, on January 1, 2017 when the extra second will be inserted at the US Naval Observatory's (USNO) Master Clock Facility in Washington, DC. 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" timescale and a second that is independent of Earth's rotation.In 1970, international agreements established a procedure to maintain a relationship between Coordinated Universal Time (UTC) and UT1, a measure of the Earth's rotation angle in space.The International Earth Rotation and Reference Systems Service (IERS) is the organisation that monitors the difference in the two time scales and calls for leap seconds to be inserted in or removed from UTC when necessary to keep them within 0.9 second of each other.