The clean answer — and the ladder above it

An hour is 3,600 seconds: 60 × 60. Keep climbing and the numbers swell fast. A day is 86,400 seconds (3,600 × 24). A week is 604,800. A 365-day year is 31,536,000 — call it 31.5 million.

There's a tidy mnemonic hiding in that last figure: the number of seconds in a year is almost exactly π × 10⁷ (31,415,926), within about half a percent. A coincidence with no deep meaning — but a handy one if you ever need to estimate a year in seconds and only remember pi.

A second isn’t a slice of an hour

It feels like the hour came first and was chopped into seconds. The opposite is true. Since 1967, the second has had its own physical definition, independent of any clock face: it is exactly 9,192,631,770 vibrations of the radiation emitted by a caesium-133 atom. The hour is then defined upward from that — 3,600 of those atomic counts.

That's why atomic clocks are the world's real timekeepers, accurate to better than a second over millions of years. The minute, hour and day are just convenient bundles of the atom's tick. The base unit is the second, and everything else is arithmetic stacked on top of it.

Why a day isn’t always 86,400 seconds

Here is where the clean math cracks. Atomic time is perfectly steady, but the Earth's spin is not — tides and the planet's churning interior make it wobble and, over the long run, slow down. So atomic time and "sun overhead" time drift apart. To keep them within 0.9 seconds of each other, timekeepers occasionally insert a leap second: a day that runs 86,401 seconds, with a clock that reads 23:59:60 before midnight.

Since the system began in 1972, 27 leap seconds have been added, the most recent on 31 December 2016. They sound harmless, but that extra tick has broken things: the 2012 leap second took down Reddit and other sites, and the 2016 one tripped up Cloudflare and a wave of Linux servers, because a lot of software simply isn't built for a minute with 61 seconds in it.

The leap second’s last decade

Fed up with the disruption, the world's metrology authorities voted to end the practice. At the General Conference on Weights and Measures in November 2022, member states resolved to abolish the leap second by 2035. After that, atomic and astronomical time will be allowed to drift apart by more than a second before any correction is considered — pushing the problem decades down the road.

There's a twist nobody planned for. The Earth has recently been spinning slightly faster, not slower, which means no leap second has been needed since 2016 — and one day we may need to remove a second rather than add one. A negative leap second has never happened, and as engineers dryly note, almost no system is ready for it. The leap second may be retired before that day ever comes.

The one unit we keep renegotiating

We treat the second as the most fixed thing in the universe — the thing every other measurement leans on. But it's quietly the most negotiated unit we have. We redefined it against an atom in 1967, we've been bolting extra ones onto our days for fifty years, and we've now voted to stop. Time, it turns out, is a standard we maintain by committee.

So 3,600 is exactly right and also a small fiction: it's how many seconds we agree an hour should hold, on a planet whose spin won't quite cooperate. For the calendar's version of the same compromise, see leap years since 1900.

Common questions

Game: seconds snap

Five quick rounds. The trick is that each unit multiplies by a different number — 60, then 24, then 7.

Sources

More about time