On New Year’s Eve 2016, some parts of the world experienced an extra second just before midnight. The “leap second” is added to keep the planet’s rotation in line with atomic clocks. Since 1972, 26 leap seconds have been introduced, but only people who work with computer systems are typically affected by the extra second. Whew! I’m glad that’s not my job!
There could be another leap year added in June of this year, but it’ll depend on if the earth’s rotation speeds up as it has been.