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Comment by quotemstr | Hacker News Reader

So what if leap seconds make the epoch 29 seconds longer-ago than date +%s would suggest? It matters a lot less than the fact that we all agree on some number N to represent the current time. That we have -29 fictional seconds doesn't affect the real world in any way. What are you going to do, run missile targeting routines on targets 30 years ago? I mean, I'm as much for abolish leap seconds as anyone, but I don't think it's useful --- even if it's pedantically correct --- to highlight the time discrepancy.

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One could imagine a scenario where you’re looking at the duration of some brief event by looking at the start and end times. If that’s interval happens to span a leap second then the duration could be significantly different depending on how your timestamps handled it.

Much more important, though, is how it affects the future. The fact that timestamps in the past might be a few seconds different from the straightforward “now minus N seconds” calculation is mostly a curiosity. The fact that clocks might all have to shift by one more second at time point in the future is more significant. There are plenty of real-world scenarios where that needs some substantial effort to account for.

It matters for some things. Without those fictional leap seconds, the sun would be 29 seconds out of position at local noon, for instance.

That does not matter at all to anyone.

Yeah. "Exact time" people are a bit like "entropy" people in cryptography. Constantly arguing about the perfect random number when nobody cares.

Did you ask everyone?

It most certainly matters to a lot of people. It sounds like you've never met those people.

For practically everyone the local civil time is off from local solar time more than 30 seconds, because very few people live at the exact longitude that corresponds to their time zone. And then you got DST which throws the local time even more off.

This is ignoring the fact that due equation of time, solar noon naturally shifts around tens of minutes over the course of the year.

To drive the point, for example local mean solar time at Buckingham palace is already more than 30 seconds off from Greenwich time.

Sure, but that doesn't mean that we invented and practise leap seconds for the sheer fun of it.

There's very good reasons that are important behind why we try and keep UTC near UT1, so saying "it doesn't matter to anyone" without even entertaining that some people might care isn't very constructive.

Okay, I’ll bite. Who does this matter to, and why?

UTC, and leap seconds, originate from (military) navies of the world, with the intent of supporting celestial navigation. It is already dubious how useful leap seconds were for that use, and much more dubious is its use as civil timescale.

We have leap seconds to save us from having leap minutes, or leap hours.

Generally, it's useful for midnight to be at night, and midday during the day. UT1 is not regular, so you need some form of correction. Then the debate is about how big and how often.

You don't need leap minutes. Nobody cares if the sun is off by minutes, it already is anyways thanks to timezones. You don't even need leap hours. If in seven thousand years no-one has done a 1 time correction, you can just move the timezones over 1 space, like computers do all the time for political reasons.

It’s going to be multiple centuries until the cumulative leap seconds add up to 30 minutes, and by that point, a majority of the human population is likely to be living off the earth anyway.

The point is, since astronomical "time" isn't exactly on constant multiple of cesium standard seconds, and it even fluctuates due to astrophysical phenomena, applications that concern astro-kineti-geometrical reality has to use the tarnished timescale to match the motion of the planet we're on rather than following a monotonic counter pointed at a glass vial.

It is up to you to keep TAI for everything and let your representations of physical coordinates drift away into the galaxy or something, but that's not the majority choice. Overwhelming majority choose UTC time.

TAI is still nice for many high precision applications, weirdly including a lot of precisely those geo-spatial use cases, so we have both.

Also, some of the most populous time zones in the world, such as the European and Chinese time zones, are multiple hours across.

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