Airlines, stock market trading floors and technology companies are braced for chaos today as scientists prepare to add a leap second to the global clock. Immediately before midnight reference clocks will read 11:59:60 for an extra second to allow the Earth's rotation to catch up with atomic time.When a leap second was last added, in 2012, Mozilla, Reddit, Foursquare, Yelp, LinkedIn, and StumbleUpon reported crashes and there were problems with the Linux operating system and with programmes written in Java.In Australia about 400 flights were grounded as the Qantas check-in system crashed.Experts at Britain's National Physical Laboratory, who will add the second to UK time, warned that markets could suffer transaction delays if their software was not prepared."There are consequences of tinkering with time," said Peter Whibberley, a senior research scientist at the lab. "Getting leap seconds wrong can cause loss of synchronisation in communications networks, financial systems and many other applications."Atomic time is constant but the Earth's rotation is slowing by about two thousandths of a second a day. The occasional introduction of leap seconds is therefore essential to ensure that our timekeeping does not drift too far from time based on the Earth's spin. ..