This guide explains what is a leap second in plain language, with the exact figures and how to work them out yourself.
Earth is an imperfect clock. Tides, molten currents in the core, even large earthquakes subtly change its spin, so the day measured by the sun drifts against the flawless beat of atomic clocks. When the gap approaches nine-tenths of a second, timekeepers insert a leap second: clocks read 23:59:59, then 23:59:60, then 00:00:00. Twenty-seven have been added since 1972, most recently on 31 December 2016.
One second sounds harmless, but modern computing disagrees. Systems that assume no minute has 61 seconds have stumbled badly on leap-second nights — websites crashed in 2012, and financial exchanges now routinely delay opening around insertions. Tech companies invented “leap smearing,” stretching the extra second invisibly across many hours, precisely because the honest approach broke things.
So the practice is ending: in 2022, the international General Conference on Weights and Measures voted to abandon leap seconds by 2035, letting atomic time drift slowly from solar time and deferring any correction to future generations — by which point the difference might be handled once a century.
A curious footnote: Earth’s rotation has recently sped up slightly, producing the shortest days ever measured and raising the once-unthinkable possibility of a negative leap second — a minute with only 59 seconds — before the retirement date arrives. No clock system on Earth has ever tested one.
Frequently asked questions
Related on itsdately: From Milliseconds to Attoseconds: The Small Units of Time · The 23-Hour-56-Minute Day: Solar vs Sidereal Time · How Atomic Clocks Work: The Machines That Define the Second
Further reading: International Earth Rotation and Reference Systems Service (IERS)