So, I only say 2016 was a horrible year because I think that every year is a horrible year, but this year was notable in that everyone else thought it was horrible, too. Well, on top of this trash year being a leap year, there's going to be one last morose second tacked on right at the end.
This is a clock
Time is meaningless, but we've somehow managed to make multiple meaningless scales that don't exactly align. First, there's Universal Time, based on the Earth's location around the Sun and in relation to the stars, which is maintained by a group called the International Earth Rotation and Reference Systems Service (IERS). Then, there's International Atomic Time, or TAI, which determines the length of the seconds, minutes, hours and so on using the ticking of at several hundred atomic clocks. UTC, Coordinated Universal Time, tries to approximate Universal Time using the TAI's ticks.
Trouble arises because Earth's rotation is irregular, sometimes speeding up and sometimes slowing down. This throws the two different systems of time-keeping out of sync. In order to keep UTC within .9 seconds of the Universal Time, the IERS will occasionally announce that everyone should add a second to their clocks. According to an IERS bulletin, those observing UTC will count the new year down as 23:59:59, 23:59:60, 0:00:00. Usually the seconds just go from 59 to 00. The added second will put TAI 37 seconds ahead of UTC.
Again, none of this really matters, because space, Earth and all things are meaningless and time is just our effort trying to add meaning to the meaninglessness. But also, your iPhone will handle the change for you.