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In page Leap second:

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Not all clocks implement leap seconds in the same manner. Leap seconds in Unix time are commonly implemented by repeating 23:59:59 or adding the time-stamp 23:59:60. Network Time Protocol (NTP) freezes time during the leap second,[1] some time servers declare "alarm condition".[citation needed] Other schemes smear time in the vicinity of a leap second, spreading out the second of change over a longer period. This aims to avoid any negative effects of a substantial (by modern standards) step in time.[2][3] This approach has led to differences between systems, as leap smear is not standardized and several different schemes are used in practice.[4]