27 extra seconds, last added in 2016
Member states of the General Conference on Weights and Measures take a proposal to end the leap second immediately to a vote at their 28th meeting outside Paris on 13-15 October. An extra second has been added to Coordinated Universal Time 27 times since the 1970s, most recently in 2016, to keep atomic clocks in step with the Earth's uneven rotation. The practice was already set to stop in 2035. The proposed replacement is a leap hour, which aligns atomic time with the Earth's rotation only once the gap between them reaches 3,600 seconds.[1]
A negative leap second has never been done
What moves the timetable now is the prospect of a negative leap second, in which clocks skip a second instead of gaining one. That has never been done. Organisations handle even the positive case differently: Google spreads the extra second across the day, some companies add a whole second to a single minute, and some exchanges shift trading hours to avoid operating during a minute lasting 61 seconds.[1]
Georgette Macdonald, director-general of the Metrology Research Centre in Halifax, Canada, describes the result as a discontinuity in a system that is meant to be continuous. Organisations that already disagree on how to add one extra second have no practised method for skipping one.[1]
A leap hour of 3,600 seconds
The proposed replacement is a leap hour that aligns atomic time with the Earth's rotation only once the gap between them reaches 3,600 seconds. Atomic time and the Earth's uneven rotation still meet; the wait between those meetings grows. The next signal is whether the 28th meeting outside Paris ends the leap second immediately or leaves the stop in 2035 standing. Pause — the second on a clock is a chosen human interval.[1]