Every so often, the world’s official clocks display something impossible: 23:59:60. That extra tick is a leap second, a deliberate one-second pause inserted into Coordinated Universal Time to let the Earth catch up. It exists because humanity runs two kinds of time at once — the exquisitely steady beat of atomic clocks, and the slightly wobbly rotation of the planet — and the two slowly drift apart.
Atomic time defines the second by a property of the cesium atom, and it does not care what the Earth is doing. The Earth, meanwhile, is an imperfect clock: tides, molten-core dynamics, earthquakes, and even shifting ice and water loads all nudge its spin rate.
The bookkeeping of a wobbly planet
Since the system began in 1972, leap seconds have been added whenever astronomical time threatened to drift too far from atomic UTC — the rule keeps the difference under 0.9 seconds. A couple of dozen have been inserted over the decades, always at the end of June or December, each giving that day 86,401 seconds instead of the usual 86,400. Crucially, they are irregular: the Earth’s slowdown is not smooth, so leap seconds are announced only months ahead by the international service that monitors Earth rotation. No formula predicts them years in advance.
Why computers hate them
A minute with 61 seconds breaks a foundational assumption baked into enormous amounts of software. Systems have historically responded to leap seconds with crashes, hung servers, and subtle data-ordering bugs, because two events one second apart can appear to share a timestamp or arrive out of order. Large tech companies developed the workaround of “smearing” — stretching the extra second invisibly across many hours so no clock ever shows :60 — but smeared and unsmeared systems then disagree with each other during the smear window, which creates its own headaches for anything comparing timestamps across organizations.
The scheduled retirement
The tension between astronomical purity and computational sanity was finally resolved in favor of the computers. International metrology bodies have agreed to stop inserting leap seconds by 2035, letting atomic and astronomical time drift apart gradually — perhaps a minute over a century or so, with the details of any future correction left to later generations. Adding to the intrigue, the Earth’s rotation has recently sped up slightly, raising the once-unthinkable possibility of a negative leap second, a minute with only 59 seconds, which almost no software has ever been tested against.
If you run systems that care about time, treat leap seconds as a live operational event until 2035 — and for everyone else, enjoy knowing that occasionally, officially, a minute really does last 61 seconds.
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