Leap seconds, and why they're being abolished

Two kinds of clock run underneath the time on your phone, and they do not quite agree. One is atomic and very nearly flawless. The other is the Earth itself, turning on its axis — and the planet is not a reliable timekeeper. For most of history the difference did not matter, because the spinning Earth was the most accurate clock anyone had. Once we built something steadier than the planet, a small discrepancy appeared between them, and the leap second is the patch that has kept the two in step for the last fifty years. It is also, as of a decision taken in 2022, on its way out.

Two clocks that disagree

Atomic time — known formally as TAI — counts seconds at a perfectly steady rate, set by the vibrations of caesium atoms. It never speeds up and never slows down. The Earth’s rotation, by contrast, is slightly irregular from one year to the next and, over the long run, gradually slowing. Solar time — the time defined by where the Sun actually sits over a given meridian, written as UT1 — therefore drifts against atomic time. The drift is tiny, a fraction of a second a year, but it accumulates, and it almost always runs the same way: the Earth falls a little behind the flawless atomic count.

UTC — the civil time standard the whole world runs on, and the thing your devices actually track — sits between these two. It ticks at the steady atomic rate, so it is precise, but it is never allowed to wander more than 0.9 seconds away from UT1, from where the Sun really is. When the accumulated drift threatens to cross that limit, UTC is pulled back into line by inserting a single extra second. That inserted second is the leap second: a small astronomical courtesy that keeps clock time and Sun time from ever parting company by more than a hair.

How a leap second is actually inserted

The insertion follows a fixed convention. A leap second is added almost always at the very end of 30 June or 31 December, on a date chosen and announced months ahead by the international body that watches the Earth’s rotation. For one second, the clock shows something that looks impossible: after 23:59:59 comes 23:59:60, and only then does it roll over to 00:00:00 of the next day. That extra tick — the sixty-first second of a sixty-second minute — is the whole of it.

The first leap second was added in 1972, when the modern UTC scheme began. Since then a total of twenty-seven have been inserted, the most recent at the end of 2016. Every one has been positive — a second added, never removed. In principle a leap second could be negative, deleting 23:59:59 entirely if the Earth ever ran fast for long enough, but that has never happened; the planet’s long-term slowing has only ever called for seconds to be put in. And because the Earth’s wobble cannot be predicted years ahead, there is no formula for when the next one falls. Each is decided case by case and announced only a few months in advance, which is part of what makes leap seconds so awkward for the systems that keep your devices on time.

When they break things

A leap second is a small kindness to the astronomers and a genuine headache for computers. A great deal of software is written on the quiet assumption that every minute has exactly sixty seconds and that time only ever moves forward at a constant pace. A minute that suddenly holds sixty-one seconds violates that assumption, and a system that was never told to expect it can misbehave. The failures are rarely in the timekeeping itself — they are in everything built on top of it: schedulers, databases, and logging systems that see a timestamp behave oddly and tie themselves in knots.

The clearest example came with the leap second added in the middle of 2012, which disrupted several large websites and online services. Some servers locked up or drove their processors to full load the moment the extra second arrived, and a number of well-known sites went down or slowed to a crawl for a time. The industry’s usual workaround now is the "leap smear": rather than insert one abrupt extra second, spread it out — slow every clock by an imperceptible amount over several hours so the second is absorbed gradually and no minute is ever officially sixty-one seconds long. It works, but it means different systems briefly disagree about the exact time during the smear, which is a mess of its own.

The decision to retire them

In the end the awkwardness outweighed the astronomy. In November 2022 the international body responsible for the world’s system of units — the CGPM, working with the BIPM — resolved to abolish the leap second, with the change to take effect by 2035. The plan is to stop adding them and simply let UTC and the Earth’s rotation drift apart, allowing the gap to grow far past the old 0.9-second limit before anyone does anything about it. The reasoning is a plain trade-off: a clock that wanders a few seconds, or eventually a minute, away from the Sun over the coming decades matters to almost nobody in daily life, while a 61-second minute matters a great deal to the machines that now run on precise time.

It is a real shift in what civil time means. For the first time, the time the world keeps will be allowed to come loose from the Sun rather than being held to it second by second. But the drift is so slow that it would take many decades to notice, so the trade is one almost no one will ever feel — and the computers, at least, will stop tripping over an extra second that was never really theirs to keep.