Leap second
A one-second adjustment used when needed to keep Coordinated Universal Time close to time based on Earth’s rotation. Every leap second introduced so far has added one second rather than subtracting one.
On a leap-second day, the last minute can have 61 seconds. To keep UTC close to Earth’s rotation, clocks may insert 23:59:60 after 23:59:59. Because that extra second can disrupt computer systems, global timekeepers plan to change the system by 2035.
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