Leap Seconds Explained: Why We Add Extra Seconds to Our Clocks

On June 30, 2012, Reddit, Mozilla, Gawker, Qantas, and many other services crashed simultaneously. The culprit? A single second added to the world's clocks. This is the story of leap seconds: why they exist, why they cause so much trouble, and why they may soon be history.

Why Leap Seconds Exist

Leap seconds exist because we have two incompatible definitions of a day. Astronomical day: one rotation of the Earth (approximately 86,400 seconds). Atomic day: 86,400 atomic seconds as defined by cesium clocks. The problem is that Earth's rotation is gradually slowing due to tidal friction with the Moon (Earth's day lengthens by approximately 1.7 milliseconds per century). This means astronomical days are getting slightly longer while atomic days stay constant. Over time, the two drift apart. A leap second is added to UTC to keep it within 0.9 seconds of UT1 (astronomical time based on Earth's rotation). Without leap seconds, over centuries, we would accumulate a deviation where midnight on the clock no longer corresponds to midnight in the sky. After approximately 1,000 years, solar noon would occur around 1:00 PM clock time.

The Leap Second Track Record

Since the system began in 1972, 27 leap seconds have been added to UTC. All have been positive (adding a second), which reflects the long-term slowing of Earth's rotation. The most recent leap second was on December 31, 2016. Interestingly, Earth's rotation has been slightly speeding up in the last few years (possibly due to changes in the Earth's core), which has postponed the need for further leap seconds. Some scientists speculate we might even need a negative leap second (removing a second from UTC) for the first time in history, though this has never been tested.

Why Leap Seconds Break the Internet

The 2012 leap second outage was not an isolated incident. Similar problems occurred in 2015 and 2016. The fundamental issue is that most software assumes a minute always has 60 seconds. When a minute suddenly has 61 seconds (23:59:60 on the leap second date), assumptions break. Some systems handle the extra second by smearing it over the course of a day (Google's leap smear approach, gradually adjusting their NTP servers' time by fractions of a millisecond). Others pause their clocks for one second, which can trigger timeouts, crash real-time systems, and desynchronize distributed databases. The 2012 Reddit outage was caused by a Linux kernel bug where a leap second insertion triggered a livelock in the kernel's timer subsystem, causing servers to spin at 100% CPU indefinitely.

The Abolition of Leap Seconds (By 2035)

In November 2022, the International Bureau of Weights and Measures (BIPM) made a historic decision: leap seconds will be abolished by 2035. The plan is to let atomic time (UTC) diverge from astronomical time (UT1) until the difference becomes large enough to justify a larger correction (perhaps a leap minute, applied no more than once per century). This decision was driven by the complexity leap seconds introduce into computing, telecommunications, and navigation systems. The exact implementation details are still being debated, but the era of leap seconds is ending. Future generations will never experience a 61-second minute.