June 21st is usually seen as the longest day of the year, but in 2015, it's June 30th that technically holds that distinction. And it could be a problem. Tomorrow at just before midnight in London, the world's atomic clocks will add one extra second, reading a time of 23:59:60 before ticking back to 00:00:00. It's a measure that's required to keep the ultra-precise devices attuned to the Earth's actual rotation, but the last time this happened in 2012, the unexpected leap second crashed Reddit, Foursquare, Gawker, LinkedIn, and a host of other sites that didn't bank on time being arbitrarily changed.
Nasdaq is simply closing early to avoid the leap second
Fortunately, most companies have factored the leap second into their operations this time around. Google has adopted a "leap smear" technique to avoid issues with the added second: rather than repeating it at the end of the day, Google's engineers will dice it into millisecond chunks and sprinkle it throughout the regular day, hopefully avoiding server meltdowns in the process. Bloomberg reports that the same process will be used by Japanese, Australian, South Korean, and Singaporean stock and futures exchanges. For others, the solution is less neat. In New York, where the leap second is scheduled to be added at around 8PM, stock markets are simply closing early to avoid headaches. Nasdaq will stop trading at 7:48PM, and shut down at 7:55PM, while both Intercontinental Exchange Inc. and CME Group are delaying data transitions during the period.