Tuesday is exactly one second longer than Monday, as the planet's timekeepers make up for infinitesimal variations in the length of the day — but apart from a few apocalyptic predictions in headlines, chances are you won't even notice.
Normally there are 86,400 seconds between one midnight and the next. That's the official length of a day — but if you actually measured it, you'd find that the average length of a day is actually 2 milliseconds longer than that: 86,400.002 seconds. This slight inconsistency is a result of the way our oceans move in response to the competing gravitational forces of Earth, the moon and the sun, continually (and nearly imperceptibly) slowing the Earth's spin.
The nice round number we go by, tracked by an atomic clock and available for everyone to synchronize with, is called Coordinated Universal Time, or UTC. The real-world measurement is called UT1. And while ignoring a couple extra milliseconds won't make you late for work, they do add up over time. Every few years we have to add up all the fractions into one whole second and stick it onto the end of a day. So 11:59 p.m. UTC will have 61 seconds instead of the usual 60.
It works great — except when it causes computer systems to crash worldwide.
How a leap can go wrong
Tom Van Baak, an engineer and clock maker who's fascinated by the leap second, explains why leap seconds are suddenly a big deal. Back in the 1950s and '60s, he wrote in an email, "maybe a couple of hundred people and machines and telescopes and room-sized computers cared. And people would flip a few switches and all would be well. No one else knew or cared."
Now there are billions of devices and systems, including air traffic control systems and stock markets, that need to know the precise time in order to function properly. The problem is similar to the Y2K bug: Some computer systems, especially old ones, just weren't designed to take this extra second into account. Since leap seconds are so rare, the bugs they create often slip past testing.
The last leap second, in 2012, crashed numerous Amazon systems, bringing down some other websites that rely on the Internet giant. Social news site Reddit also had issues. Online businesses haven't had many chances to adapt: In the formative years of the modern Web, from 1999 to 2004, there were no leap seconds at all, and there have only been three since then. This is the first leap second since 1997 to tick during stock market trading hours, further raising the stakes. But companies have learned their lesson quickly, and this year they say they're ready.
"Look before you leap," Amazon cautioned in a blog post detailing how it and others will be accommodating the extra second. Some systems deal with the issue by putting a "60" in a place that normally only counts up to 59. Some have instructed the clock to show the 59 twice. Others, like Amazon and Google, have divided up the leap second into tiny pieces and are adding them onto all the other seconds of the day, lengthening them imperceptibly and sidestepping the problem.
Not everyone is keen on this idea, since it basically amounts to temporarily and arbitrarily changing what a "second" is.
"The whole point of UTC is to allow everyone and every clock in the world to agree on what time it is," Van Baak said.