[tz] What's "right"?

Guy Harris gharris at sonic.net
Mon Nov 16 21:31:38 UTC 2020

On Nov 16, 2020, at 1:11 PM, Paul Gilmartin via tz <tz at iana.org> wrote:

>    https://datacenter.iers.org/data/16/bulletinc-052.txt
> A positive leap second will be introduced at the end of December 2016.
> The sequence of dates of the UTC second markers will be:		
>                          2016 December 31, 23h 59m 59s
>                          2016 December 31, 23h 59m 60s
>                          2017 January   1,  0h  0m  0s
> The UTC "23:59:60" is extraordinary; I'd call it the "leap second".

Yes, that's what it is.

> A "negative leap second" is problematic.  It would be a TAI second
> with no matching UTC second.

At least as I read ITU-R Recommendation TF.460-6:


if a second is removed, UTC goes from 23h 59m 58s to 0h 0m 0s the next day, so, if TAI second {year}-{month}-{day} {hour}:{minute}:{second} corresponds to UTC second {year'}-12-31 23:59:58, the subsequent TAI second would correspond to UTC second {year'+1}-01-01 00:00:00.  Yes, there would be no UTC second {year'}-12-31 23:59:59, but that's not "no matching UTC second" as I see it.

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