New Years Eve countdown this year will go… 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 1. That’s because at 23:59:59 (UTC) we will add one second to the official timekeeping clocks of the world.
From The 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.
via Boing Boing