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You may have heard Dr Shane Bergin speaking about this on Today with David McCullagh this morning. Leap seconds are being abolished and replaced by a system that will eventually allow a leap hour to be added, although that is unlikely to happen for thousands of years.
Leap seconds were introduced because a day isn't exactly 86,400 seconds long. Billions of years ago, a day was only about five hours long. According to a research article published in the academic journal Paleoceanography and Paleoclimatology, a day lasted approximately 23.5 hours during the Cretaceous Period, around 70 million years ago.
The gravitational interaction between the Earth and the Moon produces tidal effects that gradually slow the Earth's rotation. Leap seconds were added at irregular intervals to keep the time shown on our clocks closely aligned with the Earth's rotation and, consequently, the apparent motion of the Sun across the sky.
The last leap second was added on 31 December 2016, at 23:59:60 UTC. It was the 27th leap second added since 1972. The adjustments can cause problems for computer systems and other digital infrastructure, so international timekeepers have now agreed to phase them out in favour of a system that allows a much larger difference between atomic time and time based on the Earth's rotation.
In contrast, we will still need leap years because the Earth takes approximately 365.25 days to orbit the Sun, rather than exactly 365 days. Adding an extra day to February in leap years keeps our calendar aligned with the seasons. Without this adjustment, the dates of the seasons would gradually drift through the calendar, and Christmas would eventually fall in summer.
You can read more about the changes here on Scientific American.
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