Clocks of Earth will necessarily stop for a second at midnight Tuesday, June 30, as well as "keepers" of the world time will add an inline (corrective) second, consequently the final minute of the month has 61 seconds rather than 60. So at 23:59:59 GMT (2:59:59 GMT) time will "stick" to an additional second (23:59:60) before the clocks show 00:00:00 (3:00: 00 GMT) and begin Wednesday, July 1st. Other systems will just "go out" for a second.
It is the 27th time that something like that happens. Typically, our planet needs 86,400 seconds to perform a rotation of 360 degrees in 24 hours. But the Earth is receiving gravitational effects of the Sun, the Moon and other planets, so that the rotation speed is not constant but slows slightly. So in fact, the average daytime is 86400.002 seconds.
The result is that there is a gap of about two milliseconds between the natural (solar-astronomical) time and the International Atomic Time, measuring atomic clocks accurately billionth of a second.
The difference between atomic time and astronomical time is significant, and it is necessary the add of a leap second by the International Earth Rotation and Reference Systems (IERS), which is based in Germany and founded in 1987 by the International Astronomical Union and the International Union of Geodesy and Geophysics.
The IERS constantly monitors our planet and make recommendations to the International Telecommunication Union (ITU) when it is time to "stick" time. The final decision belongs to the ITU. The first correction was made in 1972. Until then, time was measured not by atomic clocks, but by the astronomical relationship between the Earth and the Sun, which was better known as "Greenwich Mean Time» (GMT).
The problem of the add of this second has intensified pressure from various parties, who demand to stop this practice, which will be considered by the ITU at a meeting in Geneva in 2015. This means that time will be based solely on individual watches without a periodic adjustment based on the astronomical time.
The consequence is that the atomic and astronomical time will diverge more and more in the future (a difference of one to two minutes over a century), so gradually the connection between the measuring time and the people's experience for the time around them will be lost… So, sometime in the future, people will see that at 8 o’ clock in the morning, the sun will go down!