Nearly 50 years ago, scientists agreed to stop measuring time using the stars, and instead use the atomic clock.
You can read more about what an atomic clock is and how it works here.
Unfortunately, the atomic clock system is almost too good. The Earth is not as accurate as the cesium atom, and actually slows by approximately two milliseconds per day.
This slowing down is caused by the Earth’s oceans, and their waves. Due to tidal systems, the seven seas actually act as a brake on the rotation of the earth.
Because the Earth spins a tiny bit slower each year, days are effectively getting longer. And that means the way we measure time needs to be adjusted every so often. So a leap second is added approximately every 500 days.
To keep the Earth and all its time zones in check, atomic clocks will be stopped for one second at 23:59 Coordinated Universal Time, on December 31 (that’s 00:59 on January 1 in CET).
This added second probably won’t be noticed by too many people, but its an important part of the process that makes sure that the measurement of time remains accurate around the world.