leap second
Sign in to saveextra second inserted or removed to keep civil time in sync with the Earth's rotation
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A leap second is an extra second that is occasionally added to our clocks to keep official timekeeping aligned with how fast the Earth is actually spinning. This adjustment matters because Earth's rotation is gradually slowing down, and without these corrections, our civil time would gradually drift out of sync with the sun's position in the sky.
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Article
Screenshot of the UTC clock from time.gov during the leap second on 31 December 2016
A leap second (sometimes called intercalary second) is a one-second adjustment occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between International Atomic Time (TAI), as measured precisely by atomic clocks, and observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation.