Leap Seconds

What are leap seconds?

About every one and a half years, one extra second is added to Universal Coordinated Time (UTC) and clocks around the world. This leap second accounts for the fact that the Earth's rotation around its own axis, which determines the length of a day, slows down over time while the atomic clocks we use to measure time tick away at almost the same speed over millions of years.

So, leap seconds are a means to adjust our clocks to the Earth's slowing rotation.

How many leap seconds have been added so far?

Since 1972, a total of 24 seconds have been added. This means that the Earth has slowed down 24 seconds compared to atomic time since then.

This does not mean that days are 24 seconds longer nowadays. Only the days on which the leap seconds are inserted have 86,401 instead of the usual 86,400 seconds.

When are leap seconds added?

Leap seconds are inserted at the end of the last day in June or December. When that is the case, UTC ticks from 23:59:59 to 23:59:60 before reverting to 00:00:00 (in the 12-hour format, this corresponds to 11:59:59 pm - 11:59:60 pm - 12:00:00 midnight). When that happens the last minute of the month has 61 instead of 60 seconds.

Who decides when leap seconds are added?

The International Earth Rotation and Reference System Service (IERS) observes the Earth's rotation and compares it to atomic time. When the difference between the two approaches 0.9 seconds, they order a leap second to be added worldwide.

Science behind Leap second

Historically, humans based time on the average rotation of the Earth relative to other celestial bodies, with the second defined by this frame of reference. However, the invention of atomic clocks — accurate to about one second in 200 million years — brought about a definition of a second independent of Earth's rotation. Instead, they're based on a consistent signal emitted by electrons changing energy states within an atom.

Earth has been falling behind the atomic time at a rate of about 2 milliseconds per day, currently trailing the atomic time by six-tenths of a second. Every now and then a leap second must be added to atomic clocks (and thus all of our clocks) to keep in sync with Earth's oddball rotation.