Atomic time NIST-F1 cesium fountain atomic clock, the primary time and frequency standard for the United States, Boulder, Colorado. (more) leap second unit of time Ask the Chatbot a Question More Actions Share Share Share to social media Facebook X URL https://www.britannica.com/topic/leap-second Feedback Corrections? Updates? Omissions? Let us know if you have suggestions to improve this article (requires login). Feedback Type Select a type (Required) Factual Correction Spelling/Grammar Correction Link Correction Additional Information Other Your Feedback Submit Feedback Thank you for your feedback Our editors will review what you’ve submitted and determine whether to revise the article.

　　External Websites Ask the Chatbot a Question Written by Erik Gregersen Erik Gregersen is a senior editor at Encyclopaedia Britannica, specializing in the physical sciences and technology. Before joining Britannica in 2007, he worked at the University of Chicago Press on the... Erik Gregersen Fact-checked by The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors. The Editors of Encyclopaedia Britannica Last Updated: Jan 21, 2025 • Article History Table of Contents Table of Contents Ask the Chatbot a Question { "@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [ { "@type" : "Question", "name" : " What is a leap second? ", "acceptedAnswer" : { "@type" : "Answer", "text" : "A leap second is a second added to the clock to resolve discrepancies between Coordinated Universal Time (UTC) and astronomical time (UT1), keeping their difference under 0.9 second. It is added at midnight on June 30 or December 31." } } , { "@type" : "Question", "name" : " Why are leap seconds added? ", "acceptedAnswer" : { "@type" : "Answer", "text" : "Leap seconds are added to keep UTC and UT1 synchronized, as Earthu2019s rotation slows and atomic clocks measure time differently. This prevents desynchronization between the two time systems." } } , { "@type" : "Question", "name" : " How is a leap second added to the clock? ", "acceptedAnswer" : { "@type" : "Answer", "text" : "When a leap second is added, the sequence of time changes from 23:59:59 to 23:59:60 to 00:00:00." } } , { "@type" : "Question", "name" : " What challenges do leap seconds pose for technology companies? ", "acceptedAnswer" : { "@type" : "Answer", "text" : "Leap seconds can cause software glitches and slow performance, as seen with companies such as Reddit in 2012. Some companies implement leap seconds by u201Csmearingu201D the second over hours or a day." } } , { "@type" : "Question", "name" : " What future changes are proposed for leap seconds? ", "acceptedAnswer" : { "@type" : "Answer", "text" : "The General Conference on Weights and Measures proposed raising the allowed difference between UT1 and UTC to more than 0.9 second by 2035, eliminating the need for leap seconds for at least a century." } } ] } Top Questions What is a leap second? A leap second is a second added to the clock to resolve discrepancies between Coordinated Universal Time (UTC) and astronomical time (UT1), keeping their difference under 0.9 second. It is added at midnight on June 30 or December 31.

　　Why are leap seconds added? Leap seconds are added to keep UTC and UT1 synchronized, as Earth’s rotation slows and atomic clocks measure time differently. This prevents desynchronization between the two time systems.

　　How is a leap second added to the clock? When a leap second is added, the sequence of time changes from 23:59:59 to 23:59:60 to 00:00:00.

　　What challenges do leap seconds pose for technology companies? Leap seconds can cause software glitches and slow performance, as seen with companies such as Reddit in 2012. Some companies implement leap seconds by “smearing” the second over hours or a day.

　　What future changes are proposed for leap seconds? The General Conference on Weights and Measures proposed raising the allowed difference between UT1 and UTC to more than 0.9 second by 2035, eliminating the need for leap seconds for at least a century.

　　leap second, a second added to the clock to resolve discrepancies between Coordinated Universal Time (UTC), which is measured by atomic clocks, and astronomical time (UT1), as measured by Earth’s rotation. Leap seconds are added to keep the difference between UTC and UT1 at less than 0.9 second. They are added at midnight on either June 30 or December 31. When a leap second is added, instead of the sequence of time being 23:59:59 to 00:00:00, the sequence becomes 23:59:59 to 23:59:60 to 00:00:00. The first leap second was on June 30, 1972. There have been 26 leap seconds since ...(100 of 671 words)

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