What Is a Leap Second?

A leap second is a one-second adjustment added to Coordinated Universal Time (UTC) to keep it in sync with Earth’s slightly irregular rotation.

This small tweak ensures our highly accurate atomic clocks don’t drift away from the position of the Sun in the sky.

Why Do We Need Leap Seconds?

Atomic clocks measure time based on vibrations of atoms and are incredibly precise. But Earth’s rotation is not constant—it can slow down due to things like tides and seismic activity.

How Are Leap Seconds Added?

Leap seconds are added by the International Earth Rotation and Reference Systems Service (IERS). When needed, the last minute of June 30 or December 31 gets a 61st second: 23:59:60.

Since 1972, over two dozen leap seconds have been added to keep UTC within 0.9 seconds of UT1 (a time scale based on Earth's rotation).

Controversy Around Leap Seconds

Leap seconds are challenging for technology systems, especially global networks, satellites, and financial markets. Some experts argue they should be eliminated.

In fact, there are plans to pause or even abolish leap seconds in the future to simplify global timekeeping.

Conclusion

Leap seconds are a subtle but important part of keeping our time system aligned with the rotation of our planet. Whether they stay or go, they remind us that timekeeping is a delicate balance between science and nature.

For more time facts, check out our Time Blog and explore our interactive tools.