The leap second event happens at the same instant around the globe.

The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation and Reference Systems Service (IERS). According to the International Radio Consultative Committee (CCIR) Recommendation, first preference is given to the opportunities at the end of June and December, and second preference to those at the end of March and September.

Our ability to measure time has become so good that we’ve run into a problem. The Earth isn’t very accurate. Which is awkward. Leap seconds are our current best effort to bridge the widening gap between Atomic Time and the actual rotation of the Earth. But the addition of a leap second causes no-end of headaches for technology systems that often prefer to ignore it.