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It’s the final countdown for the leap second, a janky way of aligning the atomic clock with the natural variation in the Earth’s rotation—but we’ll get to that. At a meeting last week in Versailles, the International Bureau of Weights and Measures (BIPM) voted nearly unanimously to abandon the controversial convention in 2035 for at least 100 years. Basically, the world’s metrologists (people who study measurement) are crossing their fingers and hoping that someone will come up with a better solution for syncing human timekeeping with nature. Here’s why it matters. 

Unfortunately for us humans, the universe is a messy place. Approximate values work well for day-to-day life but aren’t sufficient for scientific measurements or advanced technology. Take years: Each one is 365 days long, right? Well, not quite. It actually takes the Earth something like 365.25 days to rotate around the sun. That’s why approximately every fourth year (except for years evenly divisible by 100 but not by 400) is 366 days long. The extra leap day keeps our calendar roughly aligned with the Earth’s actual rotation. 

Things get more frustrating the more accurately you try to measure things. A day is 86,400 seconds long—give or take a few milliseconds. The Earth’s rotation is actually slowing down due to lots of complicated factors including the ocean tides and shifts in how the Earth’s mass is distributed. All this means that days are getting ever so slightly longer, a few milliseconds at a time. If we continued to assume that all days are exactly 86,400 seconds long, our clocks would drift out of alignment with the sun. Wait long enough and it would start rising at midnight. 

In 1972, BIMP (it comes from the French name, Bureau International des Poids et Mesures) agreed to a simple fix: leap seconds. Like leap days, leap seconds would be inserted into the year so as to align Universal Coordinated Time (UTC) with the Earth-tracking Universal Time (UTI). Leap seconds aren’t needed predictably or very often. So, instead of having a regular pattern for adding them, BIMP would tally up all the extra milliseconds and it was necessary, tell everyone to add one whole millisecond to the clock. Between 1972 and now, 27 leap seconds have been inserted into UTC. 

While probably not the best idea even back in the 70s, the leap second has become a progressively worse idea as computers made precision timekeeping more widespread. When the leap second was created, accurate clocks were the preserve of research laboratories and military installations. Now, every smartphone can get the exact UTC time accurate to 100 billionth of a second from the GPS and other navigation satellites in orbit. 

The problem is that all the interlinked computers on the internet use UTC to function, not just let you know that it’s time for lunch. When files are saved to a database, they’re time stamped with UTC; when you play an online video game, it relies on UTC to work out who shot first; if you post a Tweet, UTC is in the mix. Keeping everything on track is a major headache for large tech companies like Meta—which recently published a blog post calling for the abolition of the leap second—that rely on UTC to keep their servers in sync and operational.

That’s because the process of adding leap seconds—or possibly removing one as the Earth appears to be speeding up again for some reason—break key assumptions that computers have about how time works. These are simple rules: Minutes have 60 seconds, time always goes forward, doesn’t repeat, doesn’t stop, and so on. Inserting and removing leap seconds makes it very easy for two computers that are meant to be in sync to get out of sync—and when that happens, things break. 

When a leap second was added in 2012, Reddit went down for 40 minutes. DNS provider Cloudflare had an outage on New Year’s Day in 2017 when the most recent leap second was added. And these happened despite the best efforts of the companies involved to account for the leap second and mitigate any adverse effects.

While large companies have developed techniques like “smearing,” where the leap second is added over a number of hours rather than all at once. Still, it would make things a lot easier if they didn’t have to at all. 

Of course, that brings us back to last Friday’s important decision. From 2035, leap seconds are no longer going to matter. BIMP is going to allow UTC and UTI to drift apart until at least 2135, hoping that scientists can come up with a better system of accounting for lost time—or computers can get smarter about handling clock changes. It’s not a perfect fix, but like many modern problems, it might be easier to kick it down the line.

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