The first leap second was added to the UTC time system on the 30th June, 1972.
Title: The Future of Time: UTC and the Leap SecondAuthors: David Finkleman, Steve Allen, John Seago, Rob Seaman, P. Kenneth SeidelmannBefore atomic timekeeping, clocks were set to the skies. But starting in 1972, radio signals began broadcasting atomic seconds and leap seconds have occasionally been added to that stream of atomic seconds to keep the signals synchronized with the actual rotation of Earth. Such adjustments were considered necessary because Earth's rotation is less regular than atomic timekeeping. In January 2012, a United Nations-affiliated organization could permanently break this link by redefining Coordinated Universal Time. To understand the importance of this potential change, it's important to understand the history of human timekeeping. Read more (616kb, PDF)
The summer solstice that falls this year on June 21 marks the longest day of the year in the Northern Hemisphere, sunlight-wise. Almost imperceptibly, however, Earth's daynight cycle --one rotation on its axis--is growing longer year by year, and has been for most of the planet's history.Forces from afar conspire to put the brakes on our spinning world--ocean tides generated by both the moon and sun's gravity add 1.7 milliseconds to the length of a day each century, although that figure changes on geologic timescales. The moon is slowly spiralling away from Earth as it drives day-stretching tides, a phenomenon recorded in rocks and fossils that provides clues to the satellite's origin and ultimate fate
Fluctuations at the Earth's core subtly shift the planet's rotation by 0.4 milliseconds over a six-year cycle.Nicolas Gillet of the University of Joseph Fourier in Grenoble, France, and colleagues, modelled fluid behaviour in the Earth's core based on measurements of fluctuations in the magnetic field.The innermost region of the Earth's outer core periodically flows faster or slower, and this action "tugs" at the planet's magnetic field. Like an array of rubber bands, the field then pulls the region back towards its original position. But the effect ripples outward, changing the core's rate of rotation layer by layer.
Time is what prevents everything from happening at once.Since a year is actually 365.24219 days long, a day is added to the end of February, every fours years (except for years which are both divisible by 100 and not divisible by 400) ) to keep the calendar year in synchronization with the four seasons. When a day is added, that year is referred to as a leap year.In mankind's quest for order in the universe, we also have leap seconds to keep clocks in synchronisation with the rotating actions of the Earth.