The Town of Odyssey

Don't blink or you will miss it. Tonight we gain one second in time.

One second does not seem like much time, but that additional single second to standard time today has markets on edge.

Just before midnight Coordinated Universal Time, or Greenwich mean time -- at 11:59:59 or 7:59:59 p.m. Eastern Time -- one second will be added. What makes this so unusual -- and potentially troublesome -- is that this is only the second time that a second has been added during normal business hours.

Since nearly all financial operations are computerized, software and processes must be updated to have the correct time. "You've got to know when all the leap seconds are added or your time is going to be wrong going forward," said Eric Hunsader, CEO of Nanex, which provides market data to the financial industry. "So if somebody misses this (leap second), it's not just missing that time, they are going to be off until they get that extra second."

An estimated 10% of large computer networks could encounter problems stemming from the leap second, said Geoff Chester, public affairs officer for the U.S. Naval Observatory, the official timekeeper for the Department of Defense. "What we can say from past experience is that something will probably break -- but we don't know what," said Chester.

Markets in the U.S. are being cautious. After-hours trading on the New York Stock Exchange and Nasdaq will end at 7:30 p.m. ET, half an hour earlier than usual -- the extra second is added just before traditional closing at 8 p.m. ET. Still, the NYSE exchange "is confident that this will not cause any issues on our systems; however, we cannot guarantee the same for all systems used by our participants, market data vendors, and other third-parties," it said in a statement.

Normally, most futures and options electronic trading opens at 5 p.m. CDT on exchanges operated by the CME Group, the world's most diverse derivatives market. Tonight, its markets, including the Chicago Mercantile Exchange, will delay the opening to 7:45 p.m. CDT for many financial products.

A leap second added in 2012 led to some outages online with companies such as LinkedIn and Reddit. This time around, LinkedIn will slowly add fractions of a second -- a technique called "slewing" -- after 5 p.m. PT. Meanwhile, Amazon Web Services will add its fractions around the official leap second insertion and be caught up by noon local time.

Since this is universal time, the effect is global. Trading in Asian markets begins as the leap second arrives but each has taken steps to insert the extra time.

To maintain accurate timekeeping with the solar day, atomic clocks around the world must be occasionally adjusted slightly because the rotation rate of the Earth slows.

The International Earth Rotation and Reference Systems Service typically announces plans to add a leap second four to six months in advance. This leap second is the 26th implemented since 1972, according to the National Institute of Standards and Technology.

In the past, most have been added on New Year's Eve, "so most financial markets were closed and most business were closed and everything kind of resynced the next day," said John Lowe, NIST group leader for time and frequency services.

Software systems have to be adjusted for the leap seconds to ensure smooth operation, he said. "If you do nothing, you might have a problem," Lowe said. "I truly honestly don't anticipate problems."

Neither does Nanex's Hunsader, who expects that lessons learned from Y2K and previous leap seconds should prevent any problems. "There have been a dozen of these or so. Just because they haven't happened during market hours doesn't mean software hasn't had to deal with it," he said. "They are just erring on the side of caution."

Still, for investors he offers the Wall Street saying, "When in doubt, stay out." If you don't need to be trading at that time, "skip the hour," Hunsader said.

As for the average person, Lowe said, "they can enjoy the extra second."