Something unusual happened just before midnight on December 31, 2016.
Normally, a clock moves from 11:59:59 p.m. directly to midnight. But that night, the world waited through an extra second. For one brief moment, the official time was 11:59:60.
It was not a mistake. Timekeepers had deliberately made the final minute of the year 61 seconds long.
The reason is surprisingly simple: Earth is not very good at keeping time.
For thousands of years, humans measured a day by watching the Sun move across the sky. One complete turn of Earth became one day, which we divided into 24 hours.
Then atomic clocks arrived. These clocks measure time using the regular behavior of atoms. They are far more accurate than any clock that depends on Earth's movement.
That accuracy exposed a problem. Atomic clocks keep a perfectly steady rhythm, but Earth does not always rotate at exactly the same speed. The movement of oceans, winds and material deep inside the planet can all affect its rotation. The Moon also slows Earth down over very long periods.
The differences are extremely small — usually only tiny parts of a second. But those fractions gradually add up. Eventually, the time shown by atomic clocks can move away from the time suggested by Earth's position under the Sun.
To keep the two together, international timekeepers introduced the leap second in 1972. Whenever the gap became too large, they added one extra second to the world's official time. Twenty-seven leap seconds were added between 1972 and 2016.
Humans barely notice an extra second. Computers can find it deeply confusing.
Many computer systems are built on a simple rule: every minute has 60 seconds. A minute with 61 seconds breaks that rule. Programs may repeat a piece of information, put events in the wrong order or briefly stop working. Leap seconds have previously caused trouble for websites, computer networks and airline systems.
Some technology companies avoid the sudden jump by spreading the extra second across several hours. They make each second slightly longer, but by such a tiny amount that people never notice.
Now timekeepers face the opposite problem. In recent years, Earth has sometimes been rotating faster. If the change continues, they may eventually need a "negative leap second."
Instead of adding time, they would remove it. A clock might jump from 11:59:58 straight to midnight. The time 11:59:59 would simply disappear.
Because computers have never dealt with a negative leap second, experts worry that it could create even more problems.
One proposed solution is to stop making these small corrections. Atomic clocks and Earth's rotation would be allowed to move slowly apart. Only when the difference reached a full hour — perhaps centuries from now — would clocks receive one large adjustment.
A leap hour sounds dramatic, but it would be rare and predictable. Future societies would have plenty of time to prepare for it.
The debate comes down to one question: Should our clocks remain closely connected to the Sun, or should they follow the smooth and dependable rhythm of atoms?
Atomic clocks may be almost perfect. Unfortunately, they are measuring time on a planet that refuses to behave like one.