Today we gain a leap second. Why?

At 23:59:59 (UTC), time will "stop" as the National Institute of Standards and Technology in the US and other official timekeepers around the world add a second to our clocks. They last did this in 2012.

From National Geographic:


Just as leap years keep our calendars lined up with Earth's revolution around the sun, leap seconds adjust for Earth's rotation. This kind of fine-tuning wasn't much of an issue before the invention of atomic clocks, whose ticks are defined by the cycling of atoms. Cesium-based clocks, one kind of atomic clock, measure the passage of time much more precisely than those based on the rotation of our planet, so adding a leap second allows astronomical time to catch up to atomic time.

It's something time scientists have done since 1972, when the first leap second was instituted. In fact, researchers inserted 10 extra seconds into the world's clocks because astronomical time and atomic time were off by so much.

"World Will Gain a Leap Second on Tuesday: Here's Why" (National Geographic)