On Nov 16, 2020, at 1:11 PM, Paul Gilmartin via tz <tz@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: https://www.itu.int/dms_pubrec/itu-r/rec/tf/R-REC-TF.460-6-200202-I!!PDF-E.p... 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.