If today feels like a really long day, that’s because it is. There’s going to be a leap second this evening, making Tuesday 24 hours and one second long. The last time this happened, in 2012, a lot of computers took issue with time going all wonky and systems worldwide crashed as a result. This time around, the big businesses promise they’ve learned their lesson, and we should be able to look forward to business as normal.

The leap second is basically like a micro version of the leap year: the actual rotation of the Earth slows down in tiny increments over time. Therefore, time is not quite synced up with the way in which we mortals measure it, and so every so often we have to fudge the numbers a tiny fraction of a bit to catch up. Thus, the leap second.

However, the extra second isn’t as predictable as the clockwork arrival of February 29, and so computer systems — new and old — are not necessarily designed with it in mind. The last time a leap second arrived, in 2012, the resulting mess temporarily took down a number of sites and services, including Reddit, Amazon’s (massive, widely-used) web hosting services, and Qantas Airways. There were also ripples in the Australian financial markets, which were open at the time.

Google avoided trouble in 2012 by forcing their systems to add a tiny, tiny fraction of extra time onto every other second in the day. This year, Amazon is basically taking the same tactic.

Many of the concerns this time around have to do with computer-driven stock-trading markets. The 2012 leap second was over a weekend, but today’s is the middle of a bustling work-week. As Bloomberg points out, trading basically never stops and markets worldwide will be active — the major cities of the Asia-Pacific region are all supposed to come online right when the leap second happens.

To make sure nothing gets lost in the shuffle, many trading firms and markets are either shutting down five minutes early, or putting a +/- 5 minute “pause” around the key hour. And in Japan, South Korea, and Australia, trading will begin after the leap second.

So is everyone prepared this time around? Given the way the world works, probably not. A representative for the U.S. Naval Observatory told Bloomberg that probably about 10% of large-scale computer systems will experience a hiccup of some kind.

In the U.S., the leap second will take place around 8:00 p.m. on the East Coast (5:00 p.m. Pacific).

We Should Drop The Leap Second Before it Causes Real Damage [Wired]
With 61 Seconds in a Minute, Markets Brace for Trouble [Bloomberg]

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