2017 to arrive 1 leap second later
[...] it will be exactly one second late.
On Dec. 31, 2016, the international time keeping community will tack an additional second, known as a leap second, on to the last minute of the year.
[...] they are decreed by the International Earth Rotation and Reference Systems Service, or IERS, in Paris, which measures the Earth’s rotation and compares it with the time kept by atomic clocks.
Today, however, scientists keep track of it by aiming a network of radio telescopes at a distant quasar.
Duncan Agnew, a geophysicist at Scripps Institution of Oceanography in San Diego, explains that movements within the Earth’s liquid core can cause the spin rate of our planet to speed up or slow down.
To keep the two types of time from veering too far away from each other, the IERS calls for a leap second whenever it appears there will be more than a 0.9 second difference between astronomical time and atomic time.