When was the last leap second added, and why?

Scientists added an extra second—called a leap second—to 2008 to make up for the slowing down of Earth’s rotation. The International Earth Rotation and Reference Systems Service (IERS) in Paris, France, keeps track of time by measuring Earth’s rotation, which has been slowing down over time, and by an atomic clock, which never changes. When a difference in the two clocks shows up, the IERS adds or subtracts a second to the year. Time has been measured by the planet’s rotation for thousands of years; however, it was not until 1949 that scientists developed a clock that kept perfect time. The IERS atomic clock keeps time by viewing the fundamental vibrations of atoms. As far as scientists know, the cesium atom—which vibrates 9,192,631,770 times per second—does not change over time and is the same everywhere on Earth and in space.