Back in 1967 clocks went atomic, and human timekeeping was dependent of the earth’s rotation. The problem is, the planet is slowing down and clocks are not.
So every few years, to get everything back in sync, scientists add a second. They’ve done it 25 times since 1972. The last time was 2012, but that was on a weekend. June 30 will be the first leap second during trading hours since markets went electronic.
Leap seconds are added to Coordinated Universal Time (UTC) - and clocks worldwide - in order to compensate for the slowing of the Earth's rotation.
UTC is the time standard used to determine local times in time zones worldwide. It is primarily based on the combined output of several highly precise atomic clocks, a statistical time scale called International Atomic Time (TAI).
A normal day has 86,400 seconds, but in the atomic time scale 1 second is not defined as one 86,400th of the time it takes Earth to rotate around its axis but rather as the time it takes a Cesium-133 atom at the ground state to oscillate precisely 9,192,631,770 times.
Next leap second will occur in New York on Tuesday, June 30, 2015 at 7:59:60 PM. UTC time will be June 30, 2015 at 23:59:60.