We all know what a leap year is: Every four years, an extra day is added to February in order to keep the calendar and the seasons properly correlated. On Tuesday, a lesser known phenomenon called “leap second” made the day exactly one second longer. While it’s obviously not quite as noticeable as a whole extra day, the leap second is nevertheless an important tool to keep atomic time synchronized with the rotation of the earth.
Since 1972, only 26 leap seconds have been recorded, occurring at an interval of approximately 18 months. Unlike leap years, leap seconds are not regular. They’re tied to the speed of the Earth’s rotation, which fluctuates unpredictably due to a variety of forces. Without leap seconds, atomic time, which is used across the globe to maintain an international standard for timekeeping, would deviate from the time determined by the rotation of Earth.
“A leap second is a second added to Coordinated Universal Time (UTC) in order to keep it synchronized with astronomical time,” writes the National Institute of Standards and Technology on its official website. “UTC is an atomic time scale, based on the performance of atomic clocks that are more stable than the Earth’s rotational rate.”
Leap seconds historically occur on the last days of June or December, with the last recorded leap second having happened on June 30, 2012. Tuesday’s leap second took place at 8:00 pm EST. While the leap second is nothing more than a novelty to the average person, the subtle change has proven problematic for the finance and technology sectors. Even one extra second can cause massive issues for financial systems and computer programs.
Google does not plan to be one of the companies to suffer at the hands of the leap second. “Instead of repeating a second, we ‘smear’ away the extra second,” the tech giant posted on its Google Cloud Platform Blog last month. “During a 20-hour ‘smear window’ centered on the leap second, we slightly slow all our servers’ system clocks.” This allows Google’s internal clocks to sync up with universal time by the end of the so-called smear window.