IERS ponders reverse leapsecond...

Tony Finch dot at
Fri Aug 12 10:20:44 UTC 2022

Forrest Christian (List Account) <lists at> wrote:
> Hopefully there will be some movement next year when they're scheduled to
> discuss it again.    It's unfortunate that the first negative leap second
> is likely to occur before then.

Not that soon! There is not likely to be a leap second for 5 years or so,
based on the current projections.

The value to keep an eye on is UT1-UTC which is required by ITU TF.460 to
be between -0.9s and +0.9s; leap seconds are added by the IERS to keep it
in range. Broadcast time signals include a DUT1 value that is UT1-UTC
rounded to 0.1s precision, which must be between -0.8s and +0.8s.

DUT1 is currently 0.0s.

In the last couple of decades, DUT1 has decreased by about 1ms per day (on
average) which requires a positive leap second every few years.

In 2016, the length of day was 1.5ms greater than 24h; since then the long
term estimated LoD has been fairly steadily decreasing. It dropped below
24h at the end of 2020, and it's now 0.34ms short. (The LoD increased
slowly in the second half of 2021, but it has been decreasing all this

Depending on the threshold the IERS chooses, the current long-term LoD
estimate suggests a negative leap second some time between the end of 2026
(for a 0.5s threshold) and the end of 2029 (for a 0.9s threshold). That is
without making any more complicated predictions based on the downward
trend of the estimated long-term LoD.

These numbers come from IERS Bulletin A
analyzed by my program

My blog article from when this issue became more well known:

My other collected links on this topic

Tony Finch  <dot at>
Thames, Dover, Wight, Portland, Plymouth: Northeast 3 to 5, veering
east 2 to 4 later in Thames, Dover and Wight. Smooth or slight, but in
Plymouth slight, occasionally moderate at first in west and smooth
later in northeast. Fair. Good.

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