Hacker News Re-Imagined

Hate leap seconds? Imagine a negative one

  • 156 points
  • 12 days ago

  • @rdpintqogeogsaa
  • Created a post

Hate leap seconds? Imagine a negative one

@beeforpork 11 days

Replying to @rdpintqogeogsaa 🎙

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.


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.


@champtar 11 days

Why are we adjusting when after 100 years clock would be less than 1 min off. We should deprecate UTC and just use TAI, ~40s difference right now.


@black6 11 days

> It’s important to know that there are exactly 86400 (246060) UT1 seconds in a UT1 day. If the rotation of the Earth speeds up/slows down, the definition of a UT1 second changes since the number of seconds in a day is constant.

I recently took an introductory Latin course and learned that ancient Roman time operated on the same principle. As the days got longer in the summer, so did the absolute length of time encompassed by an hour. Hard to make a sundial to automatically compensate, I imagine.


@adolph 12 days

This is a fascinating read.

If the Earth speeds up enough, we might find ourselves pondering over the possibility of a negative leap second. According to the Time and Date folks [0], a day in 2021 is averaging about 0.2ms faster than the 84600 atomic seconds per day, ~70ms/year, so at most 14 years of this would put us over the threshold (super unlikely). In reality, we don’t have to speed up a full 1000ms of rotation speed because there was always a fractional difference in UT1-UTC.



@somishere 11 days

I'm actually fascinated by the climate change angle here. While it's already clear that our inability to tackle the climate crisis is having a physical, cultural and mental effect on our way of life .. a temporal one is something I don't think a lot of people have considered.


@fanf2 12 days

Since that was published, the earth has been slowing down gradually. I keep a casual eye on IERS Bulletin A, which has the latest earth rotation observations, predictions for the next year, and a simple formula for longer-term estimates. https://www.iers.org/IERS/EN/Publications/Bulletins/bulletin...

The line I look at is

  UT1-UTC = -0.1204 + 0.00020 (MJD - 59593) - (UT2-UT1)
which gives the current value of UT1-UTC (if -0.1204 grows to more than about +0.5 then we need a negative leap second) and the estimated long-term difference between earth’s rotation and 24x60x60 seconds (0.20 ms fast was up to 0.26 ms fast a few months ago).

Actually, I have some code that looks at Bulletin A for me, https://github.com/fanf2/bulletin-a which until recently was estimating that the next leap second would be a negative leap second in about 2028/9. But now its guess is receding into the future past 2030.

I have a few tweet threads on this topic, which you can find via https://twitter.com/fanf/status/1478767806187552776 with some clicky-clicky to dig the older ones out of twitter’s delightful user interface.

See also https://news.ycombinator.com/item?id=25145870 for some previous discussion here on this topic.

So nothing dramatic is likely to happen any time soon, though the current situation is quite weird.


@timando 11 days

Is it possible to configure my computer to run on TAI and then use the timezone to say that I'm TAI+10:00:37. When a leap second happens, treat it as a DST transition.


@gamache 12 days

Applying a negative second to a fleet is a solved problem, using the "leap smear" technique by Google. https://developers.google.com/time/smear

Applying it to millions of individual servers with different OSes? I'll make the popcorn.


@rvnx 11 days

I'm not sure why people are creating themselves problem ? UTC to be defined as the norm as it's the most stable, and those who wants to use UT1 for space / solar-related usage can use it and drift-away from UTC and that's it ?


@civilized 12 days

Now we're going to have "lies programmers believe about time":

1. Seconds in one time system last the same amount of time as seconds in another time system

2. Seconds in the same time system last the same amount of time as each other

3. Time numbers are strictly increasing - the same time number can't happen again

4. [etc, kill me]


@newpavlov 11 days

FYI you can find an extensive discussion of this article on Reddit: https://www.reddit.com/r/programming/comments/s2mij2/


@vbezhenar 11 days

I don't understand why do we need to have leap seconds. Even if time on Earth moves few minutes back, nobody would notice but astronomers. If time moves one hour back, just change your local zone offset, it happens all the time anyway, and that's about it.

It makes time calculations unreasonably complex without any benefit.


@lazyant 11 days

Martin Kleppmann, author of "Designing Data Intensive Applications" talks about this in his excellent video series (skip to 14:30 for negative leap seconds etc) https://www.youtube.com/watch?v=FQ_2N3AQu0M&list=PLeKd45zvjc...


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