On December 31, at 23:59:59 UTC, a leap second will be added to the official timekeeping clocks of the world. That's because the timescales of atomic clocks and the earth's rotation aren't perfectly in synch. The last leap second was added in 2005. From Smithsonian:
Earth's rotation is the traditional form of timekeeping. It is what defines a day. However, while we call a day 86,400 seconds, it is really 86,400.02 seconds. All those .02 seconds add up over time. In addition, the earth's rotation is not constant (it has been slightly slowing, and 900 million years ago a day was only 18 of our hours). Time as we know it changes.