I understand that technically when a leap second is added, it is added after midnight UTC, but I'm unclear how the addition is handled in other timezones. For precise reckoning of course (e.g. astronomical ephemeris), it must be added at the same instant (e.g. at 1 AM in France, or 7 PM in in New York), but for many locations doing so could be disruptive and potentially confusing. Even in Britain, the addition at midnight on New Year's Eve could be confusing, since it would either extend the countdown by a second, or require it to be started a second "late".

Are leap seconds in fact added everywhere, in all time zones, at the same UTC instant, or do different time zones (and perhaps even different countries and organizations) account for them at different, more "convenient", times.


Yes, the leap second are added at the same "instant" in the whole world, whatever is the part of the day on a given place. This is documented by a rather typical screenshot of some "clock" in 2008 according to Chicago's U.S. Standard Central Time:

enter image description here

The confusing second was added right before 6 p.m. – in as big a city as Chicago and similarly in the whole world. More comments about leap seconds and the plans to abolish them, see:


In general, leap seconds add some problems which is why there are efforts to abolish them. Still, one shouldn't overstate the problems. The amount of activities that require this 1-second precision in "absolute time" is rather limited, and even if we count trains into this set (which is a big exaggeration), the 1 second may usually be easily "lost" during the journey. More high-tech applications – like the communication with NASA's satellites etc. - are so professional and administered by so advanced people that they're expected to deal with all potential subtleties here.

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    $\begingroup$ There are costs associated with abolishing leap seconds. Moving Stonehenge each year so it aligns with druids' iphones will be a major headache $\endgroup$ – Martin Beckett Jan 1 '13 at 18:54

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