What are Leap Seconds?
A leap second is a second which is added to Coordinated Universal Time (UTC) in order to synchronize atomic clocks with astronomical time.
The reason we have to add a second every now and then, is that Earth's rotation around its own axis, is gradually slowing down, although very slowly. Atomic clocks however, are programmed to tick away at pretty much the same speed over millions of years. Compared to the Earth's rotation – which determines the length of a day – the atomic clocks are simply too accurate.
In 2015, a leap second will be added on June 30, 2015 23:59:60 UTC.
Red Hat patch
Red Hat releases patch that should help to care the Leap Second problem:
What are implications if I do not install it? Would that facilitate certain types of attacks on my system?