For the 2015 leap second, Amazon Web Services slowed its clocks by 1/86400 (.001157407%) for a day before the jump, smearing the extra second over the preceding 24 hours so its clocks would be one second slow at the time of the one-second jump. A leap second is a second added to Coordinated Universal Time (UTC) in order to keep it synchronized with astronomical time. "This extra second, or leap second, makes it possible to align astronomical time, which is irregular and determined by Earth's rotation, with UTC which is extremely stable and has been determined by atomic clocks since 1967," the Paris Observatory said in a statement. In July of this year, the International Earth Rotation and Reference Systems Service (IERS)—which regulates the international clock—announced that 2016 will end in a leap second. TAI is exactly 36 seconds ahead of UTC, a difference that keeps growing as leap seconds are added, and will reach 37 seconds on January 1. The current difference between UTC and UT1 is shown in our leap second archive web page.
– For many, 2016 can't end soon enough. For everyone, though, it's actually going to last one second longer than usual. With New Year's Eve approaching, outlets such as Phys.org are reminding people that an extra "leap second" will be added to the final day of 2016 in order to keep our earthly time-keeping measures in sync. No, this won't affect countdowns to ball drops—the second is incorporated into the day in far more subtle ways—but if you happened to watch the online clock counting Coordinated Universal Time, or UTC, you'd see the quirk of it going from 23:59:59 to 23:59:60, instead of flipping to 00:00:00. A more technical explanation: "Leap seconds are added in order to keep the difference between UTC and astronomical time (UT1) to less than 0.9 seconds," per the National Institute of Standards and Technology. Wonky, yes, but for companies such as Google that operate Network Time Protocol servers, the leap second is indeed a big deal—and a post at Popular Mechanics praises the company's "extremely elegant solution" to the issue. Google will "smear" that extra second, as it explains in a blog post. "Instead of adding a single extra second to the end of the day, we'll run the clocks 0.0014% slower across the ten hours before and ten hours after the leap second, and 'smear' the extra second across these twenty hours." Other cloud companies such as Akamai use a similar approach but over 24 hours, notes PCWorld, which expects this longer "leap smear" to be standard when the next leap second rolls around, probably in 2018. (If you're planning a trip to ring in 2017 somewhere, this list might help.)