Leap Second: How Companies Are Preparing For Extra Second Today

— -- If today feels a bit longer, there's a reason for it.

A leap second will be added to the clock to account for a discrepancy between Earth's rotation and the atomic clock.

While there have been more than two dozen instances of a leap second being added since 1972, companies relying on their computer systems aren't taking any chances.

When the last leap second was added on June 30, 2012, it caused issues with a number of websites, including Qantas, LinkedIn and Yelp.

The extra second will be added as the clock strikes midnight universal time, meaning the extra second will come for people in the United States at 8 p.m. EDT -- making it the first leap second to be added during trading hours since active 1997.

Clocks synchronized to standard civil time will show the extra second as 0:60, however it's possible that programs not equipped to handle the extra second could have an issue.

As a safeguard, U.S. stock markets are ending some after-hours trading early. The U.S. Commodity Futures Trading Commission requested plans of action last month from exchanges, according to NASDAQ.

Google explained its approach for handling the leap second in a 2011 blog post. The company has adopted an approach they call the "leap smear," which gradually adds a few milliseconds to every update to ensure when the leap second comes around, all of the systems are caught up and not disrupted.

"Our systems are engineered for data integrity, and some will refuse to work if their time is sufficiently 'wrong,'" a company blog post said. "We saw some of our clustered systems stop accepting work on a small scale during the leap second in 2005, and while it didn't affect the site or any of our data, we wanted to fix such issues once and for all."

Amazon Web Services said last month it would spread out the leap second over the course of many hours to ensure all of its systems are caught up by midnight and unaffected by the change.