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Comment by ameliaquining | Hacker News Reader

Julia is not actually doing anything unusual here. When their documentation talks about UT1, this is just a fancy/confusing way of saying that their datetime library isn't aware of leap seconds, just like every other mainstream programming language AFAIK. The abolition of future leap seconds will only reduce the number of weird edge cases this generates, which is all to the good. I suppose they'll need to change the docs, though.

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The clocks we have all basically work in TAI. Historically UTC is TAI plus occasionally a bodge (the leap second) to move it closer to UT1 when it's wrong by too much.

Nobody has a real UT1 clock†, some observation sites prepare guesses at UT1 mostly in terms of how far it deviates from TAI because you can just buy clocks that keep TAI really well.

† You could say this damp rock, "The Earth" is such a clock, and I guess I reluctantly accept that argument but then OK the UI for that clock is abysmal, nobody can use this the way you'd use an actual clock unless they accept miserably bad precision.

The Earth makes for a terribly imprecise clock, but it is the UT1 clock, or rather its defining component.

The UI is a ring laser gyroscope, right?

I’d say most ‘mainstream’ libraries pretend that every day, even a day with leap seconds, has 86400 regular-length seconds. But I thought that within UT1 the length of a second varies every day, so I would not have called that UT1.

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