Comment by beeforpork | Hacker News Reader

Comment by beeforpork | Hacker News Reader

A negative leap second would be a fun experiment. E.g. in Germany, the atomic radio clock signal DCF77 does have a bit to announce a positive leap second, but it has no way of announcing a negative leap second.

https://en.wikipedia.org/wiki/DCF77

Now, cheap DCF77 clocks (e.g., alarm clocks) don't care about the leap second bit anyway, but they will just adjust a few minutes later during normal resync. And they will do so just fine with a negative leap second. But some clocks, most probably those hidden in industrial systems, controllers, etc., may need to stay exact, and I am sure it's fun to find out which systems really care and will then fail and in what ways. :-)

Interestingly, the Japanese and the British atomic radio clock signals can do negative leap seconds:

https://en.wikipedia.org/wiki/JJY https://en.wikipedia.org/wiki/Time_from_NPL_(MSF)

For WWVB, whether negative leap seconds are supported seems to depend on whether amplitude or phase shift modulation is used.

DCF77 has a leap second warning bit. PTB says it is up to the user to know if it positive or negative. If a clock blindly assumes it is a positive leap that’s an error in the design of the clock. This is not ideal.

Likewise WWVB has a leap second warning bit. But it includes a UT1 sign bit so the user can know if the leap will be positive or negative. This design is fine and any WWVB clock or software can properly handle positive or negative leap seconds.

This is true for both the amplitude and the phase modulation formats.

It is a curious fact that UTC is specified by the ITU in recommendation TF.460 https://www.itu.int/rec/R-REC-TF.460-6-200202-I/en which also includes a specification for how radio time broadcasts should be formatted. I don’t know if any time signals actually use that format, tho.

That part of the ITU used to be CCIR, the international consultative committee for radio, which goes a little way to explaining it, but it is still strange that it isn’t the responsibility of the IAU, IERS, or SI.

Strange is what they said just after the CCIR incepted leap seconds at the SI subcommittee in charge of the subject: "The CCIR may have overstepped its remit in defining the UTC" and "The process that led to UTC may have been illogical, but it was effective"

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