Leap seconds are needed to reconcile the difference between hyper-accurate atomic time (TAI) and the slightly irregular solar time (UT1) based on the Earth's rotation. When the difference between the two approaches 0.9 seconds, a leap second is added to UTC, usually on June 30 or December 31. The sequence of time becomes 23:59:59 → 23:59:60 → 00:00:00. This practice is being phased out and will be ceased by 2035.
Why are leap seconds necessary?
Atomic clocks define the second as 9,192,631,770 oscillations of the cesium-133 atom, and this definition is incredibly stable. But the Earth's rotation isn't perfectly regular — it's gradually slowing down due to tidal friction from the Moon, and it also fluctuates due to earthquakes, ocean currents, and atmospheric changes. Without leap seconds, atomic noon would slowly drift away from solar noon, and eventually the sun would be at its highest point in the sky at what the clock says is midnight.
How many leap seconds have been added?
Since the practice began in 1972, 27 leap seconds have been added — all positive (adding a second, never subtracting). The most recent was on December 31, 2016. There has not been a leap second since then, partly because the Earth has been rotating slightly faster than usual in recent years, reducing the gap between atomic and solar time.
Why are leap seconds being abolished?
At the General Conference on Weights and Measures in November 2022, it was decided that leap seconds will be phased out by 2035. The main reasons: leap seconds cause software failures (Amazon, Reddit, and Qantas have all experienced outages due to leap second handling bugs), they can't be predicted far in advance (making scheduling impossible), and the negative leap second that would eventually be needed (if the Earth speeds up enough) has never been tested and could cause widespread issues.