Standardized time is possible thanks to the element cesium, which has a consistant oscillation that provides a reliable standard to set atomic clocks to.
And while the Earth rotates about once every 24 hours, giving us our standard day length, it doesn't rotate as precisely as cesium oscillates. This is because the planet is not a perfect sphere, so it has a slight wobble as it spins. The Earth's rotation is also affected by its fluid core and oceans - thanks to tidal action, the planet is decelerating by about 1.4 milliseconds per day each century, according to the US Naval Observatory.
In order to reconcile the two day lengths, global time-keepers can decide to add an extra second every so often to prevent the clocks from drifting off-course. Without leap seconds, the difference between atomic time and rotation-based time would separate by 1 second every 500 days.
In January, the Paris-based International Earth Rotation and Reference Systems Service (IERS), which maintains global time standards, announced that it would be adding a leap second as the clocks switch over from June 30 to July 1, Universal Time (which is 4 hours ahead of Eastern time). The last time IERS added a leap second was on New Year's Eve 2008.
The pause will be implemented automatically, according to Geoff Chester, a spokesman for the USNO.
Our clocks are adjusted by the computers that control them. Nothing physical happens to the clocks themselves, Chester said in an email.
The computers that count the seconds for the USNO's clocks are instructed to add one integral second to their real-time realization of Coordinated Universal Time, or UTC, at the end of the final minute of the UTC day, according to Chester.
So at 7:59:59pm Eastern Time, the clocks will tick off 7:59:60pm before continuing to 8:00:00pm.
Most people won't even notice the switch, but the leap second could cause minor headaches for any computers or devices that rely on the Global Positioning System, which is not adjusted for leap seconds. Some scientists have argued in favor of abandoning leap seconds, saying that the practice can muck up real-time safety systems, like air-traffic control computers that are entirely based on satellite navigation.
However, computer scientist Markus Kuhn of the University of Cambridge notes that there haven't been any credible reports about actual serious problems caused by leap seconds.
An alternative proposal to add leap hours every few centuries - the first such one would come in the 27th century - is probably not very feasible, Kuhn thinks.
Back in 1582, the pope still had enough power to cut 10 days from the calendar, but even that took half a millennium to be accepted everywhere, Kuhn wrote on his website. And that was before we had computers and liability!