Once every so often, the world's clocks pause for an extra second that does not exist on any ordinary calendar minute. That second is a leap second, and it exists because the earth's rotation and the atomic clocks that define Coordinated Universal Time do not agree with each other perfectly, and someone has to reconcile the two every few years.
Leap seconds are a small correction with an outsized reputation for breaking software, and 2035 will be the last year they are used at all.
Two clocks that drift apart
UTC is built from a network of atomic clocks that tick at an extremely stable, fixed rate. The earth's rotation, by contrast, is not perfectly steady: tidal friction, movements in the planet's core and mantle, and even large earthquakes and glacial melt very slightly speed up or slow down how long an actual day takes. Over time this means a day measured by atomic clocks and a day measured by the earth's spin slowly diverge. Historically the earth's rotation has tended to run a little slower than the atomic clock, so the gap between the two needed periodic correcting to keep UTC in step with the planet.
How a leap second is inserted
The International Earth Rotation and Reference Systems Service (IERS), which monitors the earth's rotation, announces when a correction is needed, usually with several months' notice. When one is scheduled, it is inserted at the end of 30 June or 31 December, as an extra second labelled 23:59:60 UTC, immediately before the clock rolls over to 00:00:00 the next day. For that one second, UTC clocks display a value that does not occur on any other date.
Since the system began in 1972, 27 leap seconds have been added, all of them positive (lengthening the day by a second rather than shortening it). The most recent one was inserted at the end of December 2016. No leap second has been needed since, because the earth's rotation rate has behaved differently in recent years, staying closer to atomic time than it did for most of the twentieth century.
Why they cause outages
Most computer systems assume that every minute has exactly sixty seconds and that time only ever moves forward in even increments. A leap second violates both assumptions at once: it introduces a sixty-first second into a minute, and depending on how a system's clock library handles that, it can appear to repeat a second, jump backward momentarily, or throw an exception a programmer never anticipated. Several well-documented outages at large internet platforms have been traced to leap second handling, usually because a server's internal clock briefly disagreed with itself about whether time had gone backward. Many large operators now avoid the problem by "smearing" the leap second, spreading the extra second out as many tiny fractional adjustments over a whole day rather than inserting one abrupt jump.
The decision to stop using them
In November 2022, the General Conference on Weights and Measures (CGPM) adopted Resolution 4, which agreed that the maximum allowed difference between UTC and earth-rotation time (known as UT1) will be relaxed, and that no further leap seconds will be introduced from 2035 onward, or at least the current mechanism will be retired well before the difference could grow large enough to require frequent corrections. The exact replacement scheme is still to be worked out by the relevant international bodies, but the direction is clear: UTC will be allowed to drift further from the earth's rotation than the one-second tolerance that has applied since leap seconds began, removing the need for the disruptive end-of-year adjustment.
What this means day to day
For almost everyone, leap seconds have never been visible in daily life; a single extra second at midnight on 31 December is not something a person notices. Their significance is almost entirely in software and infrastructure, where financial exchanges, satellite navigation and telecommunications networks all depend on precise, unambiguous timestamps. The distinction between UTC and GMT, and how both relate to the atomic clocks behind them, is covered in more depth in our GMT vs UTC guide. If you just want to see the current UTC time without worrying about any of this, our world clock always reflects the latest applied correction automatically.