Global timekeepers will add an extra second to the clock Tuesday night. Here's why, and what you can do with your extra second. (Gillian Brockell/The Washington Post)

On Tuesday, June 30, at a moment before 8 p.m. Eastern time, the entire planet will get a bonus second. “Leap” seconds aren't a new thing — they used to come around about once a year, starting in 1972, but they've become less common in the past few. The leap second is your periodic reminder that time as we know it is but a construct and that everything you know is a lie. Or that the Earth is gradually slowing down, anyway.

Wait, what? Yes.

“Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that,” NASA's Daniel MacMillan said in a statement.

Basically, our clocks are better at keeping time than the Earth is. Coordinated Universal Time, or UTC (which is four hours ahead of Eastern Daylight time) is based on an atomic clock, which calculates the length of a second based on (very predictable) changes in cesium atoms. It takes more than a million years to lose a second on atomic time.

[Record-smashing atomic clock is the most accurate ever]

Not so for our fair planet, which is always getting just the teensiest bit slower. In theory, the Earth takes 86,400 seconds to rotate once. In practice, it's clocking in at about 86,400.002 seconds. For shame, Earth.

Using multiple radio telescopes with a technique that was originally invented in the 1960s to take better pictures of quasars, scientists have been able to measure the Earth's orientation. (NASA Goddard via YouTube)

If we let all of those fractions of seconds pile up day after day and year after year, our standard of time would stop keeping in sync with the actual behavior of the planet. Instead, a group in the International Earth Rotation and Reference Systems Service monitors UTC1 — the time based on the Earth’s rotation, as calculated using methods explained in the video above — and throws a leap second into the mix as needed to keep UTC within .9 seconds of real-world accuracy.

Instead of switching right to July 1 at 23:59:59 UTC (which is just before 8 p.m., Eastern time) we’ll move on to the super special time of 23:59:60 first.

[No, planetary alignment won't make you float.]

We haven’t needed as many leap seconds lately, and scientists aren’t sure why. But that may change soon: The current method of measuring UTC1 is accurate to at least 3 millionths of a second, but NASA is working on a new system that’s expected to have a precision better than 0.5 millionths of a second. That will help scientists better track the long-term patterns in how the Earth's rotation is slowing — and make us more sensitive to tiny shifts in time, perhaps inspiring an uptick in leap seconds to match our clocks up with the Earth.

On the other hand, some have suggested abolishing the leap second now that we're firmly planted in the digital age. From Slate:

The system works fine for humans because we can just accept the extra second and essentially ignore it. But computers are programmed to have 60 seconds in a minute. Period. If you tell a computer that there’s going to unpredictably be an extra second in certain years, it will laugh at you. That is to say it will glitch because it can't complete its normal operations.

“The leap second is a hiccup in the time scale that’s not predictable,” said John Lowe, a group leader in the National Institute of Standards and Technology’s Time and Frequency Services division. “If you’re writing code right now you know when every leap day is going to occur all the way into the future. But leap seconds can’t be predicted. There’s five or six months of advanced notice, but that can be a problem for long-term programs that are already written.”

That … does not sound ideal. But before you panic, remember that we’ve done this loads of times already. There are software hiccups that can be attributed to leap second days — and even pretty large inconveniences — but no devastating system failures. Yet. Unless you consider the Internet being kind of broken for a few hours (as it was in 2012) to be a devastating system failure, which I very well could at 8 p.m. on a Tuesday.

But the hope is that 2012’s hurdles were a wake-up call to programmers of both essential and non-essential systems, and that more sites and services will be prepared for the extra time this year. We should have fewer problems than we did in 2012, not more of them.

I know what you’re really wondering: How should you spend your precious extra moment on Earth? Comedian John Oliver has a few ideas:

In fact, Oliver's show has purchased the Web site spendyourleapsecondhere.com to supply you with all the 1-second videos you might desire. Let’s just hope the Internet isn’t down.

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The extra second will sneak in just before 8 p.m. Eastern on Tuesday, June 30 as a way to keep up with the ever-slowing Earth. (Oli Scarff/Getty Images)