leap second

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The second that is occasionally added to Coordinated Universal Time (UTC) to keep it within 0.9 s of Universal Time (UT). This is necessary usually once, but sometimes twice, in the year. The leap second is added by extending the final minute of the day by one second either on December 31 or June 30, although leap seconds can also be added at the end of March and September if necessary.

Subjects: Astronomy and Astrophysics.

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