One can be glass-is-half-empty kind of person and be really sad about the pending leap second that will make 2016 last even longer, or a glass-is-half-full type who is thankful that Inauguration Day is a little bit further away. Or you can be a realist, and just accept the fact that at midnight UTC on New Years (7 PM EST) people living in half the world west of the prime meridian, including North America, will have an extra second of 2016. (The other half of the world will get the leap second next year).
This is due to the definition of a second based on the cycles of a caesium clock, which conflicts with the older definition of a second as 1/86,400 of a day, as calculated for an average day in 1900. The earth is now running a bit behind its time, so leap seconds are inserted as required, so that noon happens when the sun is directly overhead.
The trouble is, as David Yanofsky points out in a terrific article in Quartz, almost nobody on earth has the sun directly overhead at noon, because we broke the world up into time zones. Also, the length of the day varies considerably, thanks to the earth’s orbit not being a perfect circle and the earth itself not being a perfect sphere. Yanofsky makes a case for ditching the leap second, noting that GPS time doesn’t use it, and that it in fact might be dangerous.
A computer system that didn’t adjust for the leap second might confuse a computer that did by seemingly sending information that appears to come from the future…. a one-nanosecond—that’s one billionth of a second—discrepancy on a GPS clock will yield an inaccurate location by a foot (0.3 m), so a botched leap-second insertion into a satellite-based positioning system could be disastrous.
Some are trying to get rid of the leap second, because of the trouble it can cause. For example, Linux and UNIX do not use leap seconds in their clocks; in 2012, according to Robert MacMillan in Wired, Reddit, Yelp and Foursquare all crashed because the missed the leap second. Linux creator Linus Torvalds told Wired: “Almost every time we have a leap second, we find something.”
According to Wikipedia, the 2015 leap second crashed Twitter, Instagram, Pinterest, Netflix, Amazon, and Apple's music streaming series Beats 1, along with a few stock exchanges. Google deals with the problem by creating a leap smear, where they spread the extra second over a number of seconds before the leap. It is all complicated and ridiculous actually.
Yanofsky notes that if the leap second was eliminated, nobody would notice within their lifetimes that noon had shifted. You could get the same effective change just by moving a few miles east or west within your time zone. The campaign to ditch it appears to be gaining momentum, so enjoy it when it comes; it may be your last.