Human timekeeping relies on two main clocks, and they don’t always run at the same speed. For decades, the solution was the leap second, a small adjustment added to international time to keep atomic accuracy aligned with the Earth’s natural motion. But on Thursday, the General Conference on Weights and Measures, or the CGPM (abbreviated from its French name), voted to scrap the step-second, ending a long-running chapter in global time synchronization.
Most modern clocks rely on Coordinated Universal Time, a hybrid system caught between atomic time, which flows at an almost perfectly constant speed, and astronomical time, which fluctuates as the Earth’s rotation speeds up and slows down. To bridge the gap, UTC periodically inserted a leap second to keep up with the planet. But reconciling physical planetary shifts with precise digital systems ended up creating more technical disruptions than it solved.
break time
Computers are sensitive when it comes to timekeeping. The last few times we added a leap second to UTC – 2012, 2015 and 2016 – some computer systems couldn’t handle it. Of the three instances, 2012 was the worst, as the leap second increase caused issueds that crashed websites and software platforms, such as Reddit, LinkedIn and Mozilla.
The speed of the earth’s rotation undergoes small changes all the time. Although we cannot predict exactly how fast the Earth will rotate in the future, the conference wrote that there is a 30% chance that by 2035 we will need to add a negative leap second to keep UTC aligned with rotational time.
If adding a regular leap second was trouble, imagine what adding a negative leap second could do. The conference doesn’t want to risk finding out. So, potential future computer anomalies caused by UTC changes are an important reason why the step second goes away.
“Industrial users and the synchronization needs of national critical infrastructures, for example transportation, energy distribution, telecommunications and space projects, require UTC continuously without future adjustment,” reads the group’s resolution.
Move forward
The official end of the leap second will be May 20, 2027, when continuous UTC becomes effective. A continuous UTC means that no more leap seconds, negative or otherwise, are pending on UTC. This also means that UTC and rotational time start to drift apart.
The CGPM noted that time measurements differing by up to one hour kept UTC stable for several centuries.
This means that in the future, when it is noon, the sun cannot be higher in the sky. But when you consider that we have only added 27 leap seconds since the concept was introduced in 1972, any changes that occur in our lives will probably be very small.