On June 30th this year, a single second will be added to the world's atomic clocks, according to the Paris based International Earth Rotation Service (IERS), which keeps tracks of such things. The leap second needs to be added because as the Earth spins on its axis - causing days and nights - it is slowing down by about two thousandth of a second every day.
So, after 23:59:59 on June 30 2015, the world's atomic clocks will tick to 23:59:60 and then on to the standard 00:00:00 which is midnight. From here onwards it will continue normally to 00:00:01 which will be July 1.
The Earth's rotation, like a spinning top, is affected by many gigantic forces including earthquakes, tidal pulls and even weather. These can cause unpredictable changes in the rotation speed which is what our 'time' is linked to. The IERS tracks this, adds up bits of extra time segments and issues six-monthly bulletins to warn of any impending changes. This year's leap second will be the 26 thadded since 1972.
While such minuscule changes in time have hardly affected normal human lives till now, in this age of the computer and Internet even a second may become stunningly important. This was demonstrated in 2012 the last time a second was added to the world time. Several websites like Foursquare, Reddit, LinkedIn, and StumbledUpon suffered glitches as the extra second got added because their clocks couldn't figure out what happened.
Global IT giants are already preparing for this year's leap second. Google has devised a method called "leap smear" to get around the leap second problem. This strategy involved adding very small bits of time, adding up to one second, spread over a longer period rather than adding a full second at one go. In a blog post in 2011, Google's reliability engineer Christopher Pascoe had explained this:
"We modified our internal NTP servers to gradually add a couple of milliseconds to every update, varying over a time window before the moment when the leap second actually happens. This meant that when it became time to add an extra second at midnight, our clocks had already taken this into account, by skewing the time over the course of the day. All of our servers were then able to continue as normal with the new year, blissfully unaware that a leap second had just occurred.
While Google, with its vast resources can indulge in this fix, others may find it difficult to handle. So, you may expect some random glitches or outages come July 1.