Time stands still on Tuesday as an extra second is added to the 30th of June, 2015. Because the Earth's rotation is slowing - and in several billion years we'll all be dead - a second will be added to the clock. Without this second, we'd eventually have times of day that once were associated with the morning setting with the sun. We'd have chaos. But brought on so gradually that none would really notice the difference. Except computers. Back in 2012 when a second was added to the day, Linux-based systems were flung into chaos. Real chaos, not just imaginary.