“Because Earth is slowing down, we will synchronize our clocks by adding a leap second just before we enter 2017. Just before midnight on New Year’s Eve 2016 UTC time, a leap second will be added to Coordinated Universal Time (UTC), which is the world’s main time standard. Leap seconds are added either at the end of June or December approximately every 18 months in order to synchronize clocks worldwide with the Earth’s slowing rotation.”