Slight variations in Earth’s rotational speed sometimes cause the clocks of the world to be out of sync with Earth’s rotation. The National Institute of Standards and Technology (which maintains Coordinated Universal Time) occasionally inserts a “leap second” into the day to counter this effect. The clock goes from “23:59:59” to “23:59:60.” In 2012 the addition of the leap second caused a massive system failure at Qantas Airlines, delaying hundreds of flights.