Did you know this year will be longer than the last? 2026 will be longer than 2025, 2027 will be longer than 2026, and so on. The years will continue to grow slightly longer for the rest of time. EarthSky and timeanddate.com help us explain why.

The length of each day is determined by Earth’s rotation around its axis. We think of a day as lasting for exactly 24 hours, but every 100 years or so, the day gets about 1.4 milliseconds longer. While that seems like hardly anything to keep to track of, milliseconds can add up. 

In fact, “at the time of the dinosaurs, Earth completed one rotation in about 23 hours,” Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt, Md., told EarthSky. 

To adjust for the slowing and to keep our time standards in synch with each other, leap seconds were introduced. About every one and half years, an extra second is added to the Universal Coordinated Time (UTC) at the end of the last day in June or December. On these days, the last minute of the month has 61 seconds instead of 60 seconds. They may go completely unnoticed by most, but without them, our clocks would run too fast. 

As of this writing in 2024, the last leap second happened on Dec. 31, 2016. The International Earth Rotation and Reference System Service (IERS), has not yet announced when the next will occur.