[bloak]

Civil timekeeping is based on UTC because we want clocks and calendars to remain synchronised with the sun. It's not just astronomers who care about whether the sun is above the horizon. If you think about it, the solar day is the one unit of time that almost everybody cares about and pays attention to. If you screw up the day, you also screw up the week and anything else that is based on counting days. Why would you want to do that?

[repolfx]

But offset of UTC for TAI is like 37 seconds or something poxy like that. Nobody cares about this, the sun moves too slowly for that to make any difference.

There's been talk of setting UTC=TAI for quite a long time and I think it'd make sense. It'd take so long for TAI to drift from the actual rising and setting of the sun that we'd probably all have standardised on Swatch Beats by then anyway.

[bloak]

Yes, the difference is 37 seconds now, but it's growing quadratically. It's quite possible that we'll change the way we express times and dates, but it's almost certain that we'll want them to stay in phase with the sun, and we'll probably want to stick with the SI second and with something like TAI. My guess is we'll continue to want a simple relationship between TAI (or its successor) and the civil clock time, something like the current relationship according to which they are a whole number of seconds apart. That implies that we'll need something like leap seconds (or leap minutes perhaps). There doesn't seem to be a reasonable alternative.

[sla29970]

In 1950 astronomers pointed out that there would have to be two kinds of time, one to agree with calendar days and one to be as uniform as possible. Arguments over subsequent decades inexplicably decided that there could only be one kind of time specified by international agreements, and we ended up with a choice of two out of three characteristics in what we now call UTC. https://www.ucolick.org/~sla/leapsecs/picktwo.html

[zokier]

You website is a true hidden gold nugget, thanks for putting in the time to write all that up.

As an expert (as far as anyone around here is), what would your pick be for common civil time? Personally I feel like "precise time and simplicity" is almost obvious choice, but apparently it is not quite that clear cut.

[bloak]

"Simplicity" is rather subjective, even if you answer the question: simple for who?

Civil time has to stay in phase with the sun. As I see it, that's not negotiable. Inserting leap seconds, so that the nanosecond field of UTC remains the same as the nanosecond field of TAI, and the jumps that occur are negligible for ordinary people, seems to me overall the simplest solution, though I can see that UTC-SLS would be simpler for some people in some situations, and switching to leap minutes or leap hours would be simpler for people living now, who could then just ignore the problem. (Pollution and global warming and lots of other things can be treated in the same way, of course. Perhaps some of these things really will be easier to solve in the future, but I'd rather not rely on it.)

[zokier]

> "Simplicity" is rather subjective, even if you answer the question: simple for who?

I used simplicity in the meaning provided by link in parent comment refering to three desirable properties of time systems: "Every "day" has 86400 "seconds" (606024)."

> Civil time has to stay in phase with the sun. As I see it, that's not negotiable.

I don't see why that needs to be the case on a seconds level.

> switching to leap minutes or leap hours would be simpler for people living now, who could then just ignore the problem. (Pollution and global warming and lots of other things can be treated in the same way, of course. Perhaps some of these things really will be easier to solve in the future, but I'd rather not rely on it.)

Considering that need for leap hour would appear in over 500 years, I feel like trying predict the situation then is really borderline overarrogant.

Also leap hour would be basically a timezone shift, and I bet we will be doing timezone changes anyways in the next 500 years

[zokier]

I feel like throwing in a leap hour (basically tz shift) once in a millenia would be more reasonable solution, if for nothing else than letting future generations deal with it instead of trying to futilely pre-empt problems that are not really problems yet.

[repolfx]

What do you mean by growing quadratically, exactly? The rotational drift of the Earth is growing that fast? Where can I read more about this?

[bloak]

The length of the day is increasing roughly linearly at roughly 2 ms (per day) per century, because of tidal effects. The difference between TAI and UT1 is the integral of that, so grows quadratically. If we assume that the mean solar day was "correct" around 1900, then a hypothetical atomic clock that was synchronised to the sun around 1900 is, after x centuries, out by about 36525.x^2 ms. TAI was in fact synchronised to UTC around 1960, when atomic clocks were widely used, and the slowing down of the Earth's rotation is rather unpredictable in the short term (decades), perhaps being affected by climate change, so the numbers are all imprecise, but the long-term (centuries, millennia) quadratic growth of the TAI-UT1 difference is inevitable.

[sla29970]

Yes, roughly quadratic increase of LOD over the long term, but over short term more like a random walk. Right now the earth's crust is rotating faster than it did a century ago because things have speeded up. For a view over 2 millennia see plots of LOD at https://www.ucolick.org/~sla/leapsecs/dutc.html