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30/06/1972 Leap second is used for the first time

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What Is a Leap Second? | National Geographic

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The Leap Second Explained | Space Time | PBS Digital Studios

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Why Leap Seconds Cause Glitches

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Leap Second: Last Week Tonight with John Oliver (HBO)

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The Mysterious Leap Second

The universal world’s time was increased by one second in order to keep the super-accurate atomic clock in step with the Earth's rotation. The rotation is not regular and one day measured by atomic clock is slightly shorter than the average solar day. Hence it is sometimes necessary to correct this difference. As of June 2018, the lead second had to be added 27 times.
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