It will happen Saturday at 8 p.m.

You won't see it, hear it or feel it. But you and everyone else in the world will experience it: an extra second on the clock.

The International Atomic Time, which is calculated from the average of the time as measured by 50 atomic clocks around the world, is being adjusted by one second at 7:59 p.m. ET on June 30, explains Joe Rao at Space.com.

We last experienced a "leap second" like this one on New Year's Eve 2008, and it's time for another.

These occasional "leap seconds" are needed because the rotation of the Earth is very gradually slowing over time. As a result, the Earth's physical days are getting about two milliseconds longer each day, while our super-accurate atomic clocks—they would lose only one second every 200 million years—keep ticking along.

So to make sure the world stays in sync with, uh, the world, the atomic clocks need to be paused once in a while to let the Earth catch up. Right now, the Earth is about six-tenths of a second behind. This will put it four-tenths of a second ahead.

Because the rate of the Earth's rotational slowdown is irregular, we don't yet know when the next leap second will be.

Make this one count.