There is a good reason why the days seem to drag on: the earth’s rotation is actually slowing down. To reflect this change, the time lords that manage Coordinated Universal Time (UTC) occasionally add a leap second on a predetermined day, hour, and minute: the last leap second occurred at 23:59:60 UTC on June 30, 2015. This small adjustment mostly went unnoticed, but the practice itself has sparked fears over the stability of the world’s timekeeping systems. The time lords of the International Telecommunications Union (ITU) discussed leap seconds last month at the World Radiocommunication Conference in Geneva, Switzerland. Ultimately, ITU delegates decided to postpone the leap second debate for another eight years. The reluctance to decide the issue shows how small adjustments to timekeeping can have big consequences for a world increasingly dependent on technological precision.
The irregular addition of leap seconds presents a problem for accurate timekeeping by today’s computer networks. A leap second added in 2012 caused several popular websites such as LinkedIn, Reddit, and Yelp to crash. Hundreds of Qantas passengers were delayed because the airline’s flight records system also went down. While glitches persist, other groups have developed novel solutions for the problem. Google adds batches of milliseconds to its digital clocks before a scheduled leap second, a practice known as “leap smear”. The Global Positioning System simply ignores leap seconds and broadcasts a correction to its receivers. These fixes prevent a Y2K-style breakdown from occurring with every added leap second.
UTC has gained 26 leap seconds since their introduction in 1972. These leap seconds account for imperfections in the earth’s rotation caused by tidal and tectonic activity. Movements of the earth’s oceans, crust, mantle, and core change the planet’s distribution of mass. Just as a figure skater positions her arms to spin faster or slower during a routine, the earth’s distribution of mass also affects its rotational speed. Eventually the most accurate of clocks no longer reflect solar time, so leap seconds are added periodically to synchronize UTC with the solar day.
Scientists at the International Earth Rotation and Reference Systems Service add leap seconds after the fact because they cannot predict changes to the earth’s rotation ahead of time. This top-down approach to solar time contrasts with the bottom-up method used in everyday precision timekeeping. Atomic clocks define a second as the amount of time it takes for a cesium-133 isotope to oscillate 9,192,631,770 times as measured by a laser. The “cesium standard” provides a reliable way to code time into computer systems. Arbitrarily adding a leap second requires many time systems to be updated manually, a task at odds with the convenience of automatic timekeeping for computers.
Disruptions to computer systems have led to calls for the abolition of leap seconds. The United States and China want to abandon the leap second to ensure the smooth operation of time systems, while the United Kingdom and Russia favor the solar standard. The UK’s Royal Greenwich Observatory sets the solar-based Greenwich Mean Time that divides the world’s time zones, while Russia’s GLONASS navigation system still incorporates leap seconds. However, both the U.S. and UK already depart from solar time during Daylight Savings Time. Given the politics of timekeeping, the fate of leap seconds may rest on history and custom as much as scientific accuracy or convenience.