Enjoy your leap second!
The adjustment is needed to reflect a slowing in the Earth’s rotation which gradually prolongs the solar day. The day has not been exactly 24 hours long since 1820. When dinosaurs roamed the planet, the day was only 23 hours long.
The slowing of the earth did not matter much as long as time was measured in accordance with the average rotation of the Earth relative to other celestial bodies. Modern atomic clocks, however, are based on a consistent signal emitted by electrons within an atom. They are accurate to within about one second in 200 million years.
One challenge is that the slowing of the Earth is irregular, influenced as it is by the gravitational pull of the sun and the moon and by unpredictable changes in the atmosphere and in the planet’s molten core.
As a consequence, the scientists charged with measuring the Earth’s rotation determine the timing of leap seconds only six months in advance.
The extra second will occur at 7:59:60pm CST — that is, between 7:59:59 and 8:00:00.