The people responsible for measuring the world's time have got very good at determining just how long a second is supposed to last—the most accurate clock in the world uses cesium atoms to determine the exact length of a second, and it won't get out of sync for at least 300 million years.

But the question of how many seconds are in a year is far less certain. The International Earth Rotation and Reference Systems Service, which is responsible for Coordinated Universal Time (UTC), decides twice a year whether or not a "leap second" is needed to ensure that the world's clocks are in sync with the Earth's rotation, and this week its scientists decided that 2016 needs one of those extra seconds.

It will be added at midnight on Dec 31, when clocks will read 11:59:59 p.m., then 11:59:60 p.m., before the stroke of 12:00:00 a.m. ushers in the year 2017.

The addition of a leap second to the clock is a catch-up move based on the observation that the Earth's rotation trails the time found on today's atomic clocks by around six-tenths of a second. The rotation slows down relative to atomic time in tiny but non-uniform increments because of the braking action of the ocean tides.

That slowdown is roughly equivalent to a loss of around two milliseconds per day, so the Paris-based IERS evaluates whether or not to add a leap second twice per year, on June 30 or December 31. As the US Naval Observatory explains, "[a]fter 500 days, the difference between the Earth rotation time and the atomic time would be one second. Instead of allowing this to happen a leap second is inserted to bring the two times closer together."

A leap second has been added 26 times since the practice began in 1972, according to the observatory.