Yet another Hacker News

41 points Asmod4n 4 days ago 48 comments

It’s been 10 years since we had the last leap second and it looks like we will get the first negative one soonish. Are systems ready for that?

d00d0ff000 4 days ago | parent

NTP.

By any other standard, most manually set clocks are up to a full minute off all the time.

subscribed 4 days ago | parent

Yeah, but we're thinking of systems where nanoseconds matter.

MiFID 2 alone forces sub-μs precision. Million times less than the leap 1 second.

NTP minute away is good for displaying date on the workstation, not for many of the devices that are critical to the modern world.

cyanydeez 4 days ago | parent

dont most systems that rely on sharp timing simply manage it themselves.

subscribed 4 days ago | parent

Yesno.

Sure they have their own time servers fed from the GPS, but they need to be _accurate_ in relation to the world.

But timestamps used by companies forced to use very accurate timing must be synchronised to UTC.

dmurray 2 hours ago | parent

MiFID 2 does not require nanosecond accuracy. It's something like 100 microseconds in the strictest case.

Some MiFID reports require microsecond or perhaps nanosecond precision, but that's really just a formatting requirement "please write your timestamps with six figures after the decimal point."

wmf 4 days ago | parent

Systems are absolutely not ready. Leap seconds are a bad idea and negative leap seconds are worse. Just don't do it and let the drift cancel out.

toast0 3 days ago | parent

negative leap seconds aren't too bad. jumping forward a second won't lead to a time loop like jumping back did on several systems (some twice!)

knorker 2 hours ago | parent

What's worse about negative leap seconds? The "experienced" time by systems will just look like they froze for a second. Added leap seconds are worse, surely, as time goes backwards.

gmuslera 2 hours ago | parent

Suppose being charged by time used to run your tasks. Then your task takes -1 second, you will be charged zero, 1 second, or 18 quintillon seconds?

flexagoon 1 hour ago | parent

If you're calculating time that a task took by subtracting two UTC datetimes instead of just using a monotonic timer or at least unix timestamps, you're doing it wrong either way

voxic11 2 hours ago | parent

Explain why negative leap seconds are worse? Intuitively it seems like normal leap seconds would cause way more issues, or at least not more.

Bender 4 days ago | parent

Google's proposal is a smear. [1] Most time servers do not use smear. No idea what behavior it may introduce in places where sub-second time is important. Curious if all these bugs [2] were fixed specifically to deal with going backwards.

[1] - https://developers.google.com/time/smear

[2] - https://rivassec.com/leap-second-chaos-2012.html

yen223 3 days ago | parent

The brilliant thing about the smear is that it distributes the new second across each second of the day, so that each second differed by 1/86400 seconds, well within the margin of error for NTP.

As far as the computers were concerned, nothing was different.

toast0 3 days ago | parent

The less brilliant thing about the smear is that if your ntpd syncs from smeared and unsmeared servers, the results aren't great.

It would have been better if they would have kept the time on the wire accurate or added mandatory protocol stuff to avoid confusing things for ntpds configured to different leap second handling.

not-a-llm 3 hours ago | parent

if you need below 1 millisecond time accuracy, probably you know what you are doing and you wont mix NTP servers (and I think you need PTP for that)

chaps 2 hours ago | parent

You'd think so. Legacy servers that've been running for ten years really puts a hitch in that.

ikiris 2 hours ago | parent

User error

ikiris 2 hours ago | parent

Yeah 90% of the time the simple solution is just use Google time and these problems are smeared away because they got burned enough internally they did it themselves

al_borland 3 days ago | parent

If we have positive and negative leap seconds, why are we doing anything at all? 1 second forward, just to go 1 second back 10 years later…

yen223 3 days ago | parent

I don't think we can predict ahead of time whether we'll need a leap second or not

If the question is "why bother syncing time to Earth's orbit around the sun at all", I don't have a good answer for that except at this point, it's tradition.

jMyles 3 hours ago | parent

We can set some rasterization floor, such as like 3 minutes or something, and live with that.

Correcting for a 3-minute offset every few millenia seems easier than trying to understand all this minutia about wobble and aquifer management and whatever else goes into a leap second.

StevenWaterman 2 hours ago | parent

I'm pretty sure that's the plan. Currently they're legally bound to keep it within 0.9s of solar noon (or something like that) but in 2035 it's changing to +-1 minute, which basically kicks the can down the road for another century or so

I say we let it reach 15 minutes then countries can solve it themselves by shifting timezone by 15 mins. Since making sure solar noon matches noon on the clock, is the point of timezones existing in the first place

happytoexplain 3 hours ago | parent

The Earth's rotation is slowing down in the long term, hence the need to adjust. In the short term (where 10 years is "short") it can speed up or slow down, but long-term it is slowing down.

Note that this is not an argument for leap seconds - just my understanding of their rationale.

dgrin91 3 hours ago | parent

I wonder how many systems actually care? I presume the core NTP servers handle this well, and most systems just feed off of that?

GPS satellites probably handle it well too, but maybe some consumer or even industrial GPS receivers don't? Maybe some trading systems? I don't think crypto systems care too much.

not-a-llm 3 hours ago | parent

traditional markets are closed when time changes happen

TrueDuality 3 hours ago | parent

The problem frequently crops up in order-deterministic systems that use time and haven't accounted for the edge case of all the vagaries related to time-keeping of this being only one.

I've worked on some extremely sensitive systems that had thousands of lines of C dedicated to handling skewing a time gap across an hour-per-second when necessary. I know that code assumed only "missing" time (jump-forwards)... Even knowing what I know as a developer now, if I was re-implementing that system from scratch and didn't have this top-of-mind, I'd bet I would miss "overlapping" or "duplicate" time entirely.

Maybe that is more of a me problem than others, but I'd bet there are some safety critical systems out there where the responsible engineers, QA, and specs all missed this as well.

netsharc 3 hours ago | parent

I wonder if there's things that run 24/7 and need to be monitored.. e.g. if you have oil flowing through some pipeline at 100 liters/second, one particular minute will have 6100 liters, and someone will want to get paid for that 100 extra liters.

But the meter/reporting tool would say "Well, we measure every second, and the meter reported a constant rate of 100 liter/second, and as we know we have 60 seconds in a minute, so we got 6000 liters!".

Or a database for "measurements every second for this minute" that has 60 fields, and don't have a field for the 61st measurement.

blaze33 2 hours ago | parent

I once worked with smart meters for electricity consumption that do run 24/7. Leap seconds were not an issue but we had a very similar one now that I think about it: DST shenanigans!

Like how much time is there between 2 and 3 am? Usually one hour, but sometimes 2 and also sometimes 0. It looks simple at first but it creates a lot of edge cases that your business logic now needs to handle and we had a fairly complex system for this.

wat10000 2 hours ago | parent

GPS uses its own time base that doesn't do leap seconds. For display purposes, the leap second offset to UTC is transmitted to the receivers and added to the displayed time if needed.

chaps 2 hours ago | parent

Systems definitely care, especially in finance and trading systems.

Was involved in rolling out a large NTP annealing patch about ten years ago. We missed a couple and the effect was largely overall muted, but we did have one server with an old JVM hard crashing the server right at the second shift.

That specific server was already hobbling along so it wasn't a surprise. But it required a bit of firefighting.

aomix 3 hours ago | parent

Not responsible for those systems.

The last time this came up I thought “smearing” the second over the course of a day kind of solved the problem a discrete +/- 1 second suddenly appearing on the clocks.

deepspace 3 hours ago | parent

Wasn't there a recent discussion here, where it was pointed out that leap seconds are about to be phased out in less than 10 years' time? I would be extremely surprised if a negative leap second was implemented before then, given that IERS already balked at doing that several years ago.

thomashabets2 2 hours ago | parent

I'd say yes we are ready. gettimeofday() should never be used to measure time[1], but at least with a negative leap second it's monotonic.

We'll just get some poorly coded stuff claim that an operation took 1100ms instead of 100ms. Not great, but not -900ms.

Well, I say that, but per my link here F5 load balancers at least used to keep track of TCP connections using gettimeofday. And it's annoying that libpcap delivers metadata in wallclock time.

[1] https://blog.habets.se/2010/09/gettimeofday-should-never-be-...

happytoexplain 2 hours ago | parent

> gettimeofday() should never be used to measure time

And yet, even as somebody who has no idea what platform you're referring to, I can still guarantee you that gettimeofday() is used to measure time on that platform. This is how software works, unfortunately.

thomashabets2 1 hour ago | parent

As per my linked blog post…

ivanjermakov 1 hour ago | parent

BadBadJellyBean 2 hours ago | parent

Since the whole leap second system will be phased out by 2035 anyways I doubt that anyone will test it. No need to rock the boat over a second.

OsrsNeedsf2P 2 hours ago | parent

Don't systems regularly have this issue when they do time server syncs?

SeanAnderson 2 hours ago | parent

I don't think we're going to do any negative leap seconds.

https://www.yahoo.com/news/science/articles/international-ti...

I think we're voting to change to a leap hour in early 2027. Or I'd assume we're going to go that route instead of continuing to entertain the tech nightmares.

ssl-3 2 hours ago | parent

We've on track to do something different before the end of 2035: https://en.wikipedia.org/wiki/Leap_second#Phase-out_and_futu...

Shifting to a leap-minute feels close-enough to me: We might get one every 50 or 100 years. A lot of us reading this today will never live to see a leap-minute, but it's close enough that we'll still have it collectively in-mind when it it needs to happen. (And if we screw it up at that time, the outliers will only be off by a minute. Not so bad.)

A leap-hour, meanwhile: That kicks the can so far down the road that we'll probably lose track of it completely. ~6,000 years is a very long time; society will be a very different thing by then. Leap-hours seem to me to be moral equivalent to the "fuck it, let's just give up" option.

edit: accidentally a word, and fixed an off-by-an-order-of-magnitude error on the approximate years required for a leap hour

DSMan195276 1 hour ago | parent

> A leap-hour, meanwhile: That kicks the can so far down the road that we'll probably lose track of it completely. 600 years is a very long time; society will be a very thing by then. Leap-hours seem to me to be moral equivalent to the "fuck it, let's just give up" option.

Something to consider: The use of timezones in mostly 1-hour increments over inconsistently placed areas means that the vast majority of people are already living many minutes off from the "actual" time at their precise location, in some cases even more than an hour. "Giving-up" implies that this is something important worth maintaining, where-as for the vast majority of people they gain nothing from leap seconds or even leap minutes. The most important thing for people is simply that everybody agrees on what time it is, which is easier when leap-Xs aren't done.

That said it's also probably true that a leap-hour would never actually happen, but that's not some big issue. By the time we got to the point that a leap-hour would make sense people would have already adjusted their habits and it probably wouldn't be worth it.

schoen 1 hour ago | parent

An extreme example is Xinjiang, where solar time can be more than three hours off from civil time (due to the PRC policy of having only one time zone for all of China).

ssl-3 1 hour ago | parent

Indeed. On a long-enough timeline, people will adjust to whatever it is that the sun is doing regardless of what the clock on the wall says. That's the way it has always been.

So when we're talking about one second every once in awhile, I'm not sure that [effectively] giving up by adopting leap hours instead of leap seconds isn't the right option -- as long as we agree to do it uniformly.

throw0101a 1 hour ago | parent

The FreeBSD folks test their code for these things and it works:

* https://lists.freebsd.org/pipermail/freebsd-stable/2020-Nove...

Of course third-party userland code understanding what happens is another thing.

jaggederest 1 hour ago | parent

I'm rooting for pure UTC never adjusted to actual sidereal days. Let me wake up at 1600 and go to sleep at 800, it's fine.

_alternator_ 1 hour ago | parent

You may be relived to know that Unix epoch time does not have this problem. But you may also be horrified to know why.

First, epoch seconds are not the numbers of seconds since 1970/01/01. This is a lie we tell to children. Rather, epoch seconds are the number of days since 1970/01/01 * 86400 plus the number of seconds since midnight.

Leap seconds, to epoch time, don't exist. Or maybe they are double counted. Or maybe we smear them over 12 or 24 hours (but which 12 or 24 hours depends on whether you are Google, Microsoft, or Oracle; I can't even make this stuff up). The point is, it's not defined, and this means implementations do it differently.

A negative leap second might be easier though. The spec suggests (though lack of speaking) that a correct implementation will just skip it since number of seconds stays less than 86400 for that day. But of course the smear-organizations still smear it.

So what if you really want to know how many seconds were between two different epochs? Subtracting epoch seconds is wrong because you need to correct for the number of leap seconds between the two times.

And the smears.

SahAssar 1 hour ago | parent

Yes, it's wrong. But we have also agreed to all be wrong in the same way. Except during the specific day that we are wrong in different ways.

Correction: We are mostly right, most of the time, but wrong in ways most people don't notice except if they try to talk while everyone is wrong.

Clarification: Human perception of time is not understandable, and the machine abstraction even less so.

FabHK 25 minutes ago | parent

Indeed. I like to say, we would like three things to be true:

1) Every day has 86400 seconds.

2) Every day is from noon (sun exactly above) to noon (sun exactly above).

3) We use SI seconds.

You can't have all three. Pick any two:

1, 2, not 3: What you describe. Day has 86400 seconds, we keep in sync with the sun, but we tweak the seconds a bit. There are different versions, like epoch time, or UT1.

1, not 2, 3: Every day has 86400 SI seconds, and we slowly go out of sync with the sun. That's TAI.

not 1, 2, 3: We use SI seconds and keep in sync with the sun +/- 1 second, but need to add/drop seconds occasionally. That's UTC.

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

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

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