NASA has explained that 30 June 2015 will officially be a bit longer than usual because an extra second or “leap” second will be added.
Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt said that Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that.
A day lasts 86,400 seconds. That is the case, according to the time standard that people use in their daily lives, Coordinated Universal Time (UTC). UTC is “atomic time,” the duration of one second is based on extremely predictable electromagnetic transitions in atoms of cesium.
These transitions are so reliable that the cesium clock is accurate to one second in 1,400,000 years.
However, the mean solar day, the average length of a day, based on how long it takes Earth to rotate, is about 86,400.002 seconds long. Scientists estimate that the mean solar day hasn’t been 86,400 seconds long since the year 1820 or so.
The leap second will be added to June 30 at 11:59:59 UTC on the dot. What this means is that, rather than switch to a brand new day, the atomic clocks that scientists rely on to keep track of time will also show 11:59:60 UTC.
The reason some days need be made to last 86,401 seconds instead of just 86,400 is because otherwise atomic clocks might become out of sync with Earth’s rotation.
But for this extra second, the Coordinated Universal Time measured by atomic clocks could over the years become so out of sync with Earth’s rotation that it would show noon instead of midday.