This will be the 26th leap second added to a calendar year since the practice began in 1972. In the past, the extra second has messed with computer systems. The last leap second was added in 2012, and it caused problems for big companies like Reddit, LinkedIn, Gizmodo and FourSquare. The problem is that during the leap second, the computer clock shows 60 seconds instead of simply rolling over to the next minute, or shows the 59th second twice. The computer sees a leap second as time going backward, Matsakis said. The machine registers this as a system error, and...