No leap second will be added tonight

http://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat

by ColinWright • 12 years ago

136 76 12 years ago

hpiers.obspm.fr

izzydata• 12 years ago

Thank you for the service announcement. I will continue to go about my day expecting the second to not be 1 off.

Edit: No need to get so worked up over a joke people.

bpicolo2• 12 years ago

These things matter a lot for time libraries.

tomp• 12 years ago

But probably not at such short notice.

rplnt• 12 years ago

Yes, they do. However, adding the second is not the default, so announcement that it's not going to be added is not as interesting. Also, I doubt "time libraries" "people" get their news from HN, on a day it could happen.

nawitus• 12 years ago

It's pretty "crazy" that all software that handles UTC needs to have an access to the internet (or software updates) every 6 months. You simply can't leave, say, a sensor somewhere for a year and expect it to record events in UTC.

Sure, you can record events in something else than UTC and convert it to UTC at a later time.

gee_totes• 12 years ago

I did not know that the speed of the Earth's rotation is affected by earthquakes[0] and the weather[1], and thus the need for a leap second.

0:http://en.wikipedia.org/wiki/Earth's_rotation#Changes_in_rot...

1:http://en.wikipedia.org/wiki/Fluctuations_in_the_length_of_d...

datenwolf3• 12 years ago

Another important effect is the transfer of angular momentum of the Earth to orbital momentum of the Moon by tidal forces.

srean• 12 years ago

Poul-Henning Kamp's article "The One-second War (What Time Will You Die?)" [0] is a great read. I certainly was unaware of the background, and how it can affect actual systems.

[0] http://queue.acm.org/detail.cfm?id=1967009

deisner• 12 years ago

Another great article on the topic, from American Scientist: "The Future of Time: UTC and the Leap Second", available either from the magazine (behind paywall): http://www.americanscientist.org/issues/feature/2011/4/the-f... or on arXiv.org: http://arxiv.org/ftp/arxiv/papers/1106/1106.3141.pdf .

JadeNB• 12 years ago

I'm not sure about the etiquette of arXiv links, but I have always preferred being taken to the abstract, so that I can decide whether I want the paper: http://arxiv.org/abs/1106.3141. It's just a click from there to the PDF (or other formats, if there are any).

vxNsr• 12 years ago

I'm getting a 500-server error

markbnj• 12 years ago

That's a great article. Thanks for linking it.

kintamanimatt2• 12 years ago

Is there a reason why a leap second wouldn't be added if the difference between TAI and UTC is 35s?

fsk• 12 years ago

35s is the cumulative sum of all leap seconds so far

kintamanimatt2• 12 years ago

Follow-up question: what's the condition for a leap second to be added?

oofabz• 12 years ago

It sounds like the leap seconds all have the same sign. If corrections are always being applied in the same direction, that implies a second is not as close to 1/86400 of a solar day as it could be.

Maybe we should redefine the second. It is currently defined as "9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."

9,192,631,770 * 35s / 40y ~= 255. If we adjust the definition by 255 periods, TIA and UTC would stay in sync much better.

taejo• 12 years ago

Leap seconds are added to UTC to keep it within one second of UT1; the drift from TAI is unbounded.

In TAI, each second is the same length, and each day lasts the same number of seconds; UT1 is (conceptually, approximately) the solar time in Greenwich, so the length of seconds depends on the position of the earth; UTC has seconds of fixed length, like TAI, but occasionally has more or (potentially) fewer seconds in a day, to keep synch with UT1.

If you used TAI, eventually the sun would not be overhead at noon -- and after many millennia, the sun would be up during the "night" and down during the "day" (after around 70 000 years if leap seconds continue to be added at the same rate, but the whole point is that they aren't -- at the moment they seem to be accelerating).

ggreer• 12 years ago

Thank you for explaining the various time standards concisely. I think a lot of people don't know about TAI, but would prefer it to UTC if they did. Leap second handling has been responsible for some pretty serious software bugs. It's so bad that Google doesn't use them internally.

I think it makes the most sense for computers to use TAI internally, and have UTC as a time zone on top of that. Whenever there's a leap second, just push out a TZ update. Most applications don't have special code for time zone updates, which means a lower likelihood of bugs.

Using TAI internally would have another advantage: Computers with GPS receivers could just add 19 seconds to GPS time to get a very accurate TAI. (GPS time is defined to be 19 seconds behind TAI. That was UTC in 1980.)

ThrustVectoring2• 12 years ago

Defining time based on "sun directly overhead at a particular place at noon" is, itself, a holdover from navigation and seafaring. The time difference between when the sun is directly overhead where you are and directly overhead at a particular place is the latitude difference between the two places.

That's the historical reason why we have a time definition that is locked to the rotation of the Earth. Importantly, anyone navigating based on that principle will get skewed answers if when noon is at a particular place changes.

sampo• 12 years ago

after around 70 000 years if leap seconds continue to be added at the same rate

Apparently it's only 3000 years:

"Leap seconds are not a viable long-term solution because the earth's rotation is not constant: tides and internal friction cause the planet to lose momentum and slow down the rotation, leading to a quadratic difference between earth rotation and atomic time. In the next century we will need a leap second every year, often twice every year; and 2,500 years from now we will need a leap second every month.

On the other hand, if we stop plugging leap seconds into our time scale, noon on the clock will be midnight in the sky some 3,000 years from now"

http://queue.acm.org/detail.cfm?id=1967009

derefr• 12 years ago

A great little bit of fiction, if you want to revel in the sense of how weird it is that our measures of time are effectively "by fiat": http://slatestarcodex.com/2013/11/03/the-witching-hour/

marcosdumay• 12 years ago

Dates are decided by fiat (always with a bias of keeping astronomical events at sync), timespan is very well defined and does not change at all.

aaronblohowiak• 12 years ago

Elapsed time does not change, but the current time is. We measure elapsed time based on a presumably stable measure -- the vibration of cesium under exposure to microwaves -- but we want 'now' to be based on the earth's rotation and location with respect to the sun, which is gradually decaying. Atomic vs Astronomical time.

lucb1e• 12 years ago

Was hoping to read something that would blow my mind, but didn't :(

TL;DR for those who want to read it but don't really have the time: in the future some civilization only has a few objects from our present time left, one of which is digital clocks. At some point they noticed them going forwards in spring and backwards in autumn (DST presumably, but they don't know that). They think the previous civilization (us) modified time because the clocks are out of sync with the sun clocks and the stars. So it must be that we created an hour of our own: nine thousand hours a year from gods and one hour from men. The gods noticed what we did, killed us all and that's why we're no longer.

melloclello• 12 years ago

It's a cool story. How did you manage to read it and come out jaded?

mrgriscom5• 12 years ago

Can anyone explain why leap seconds exist vs. a gradual skewing of atomic time to match the earth's rotation. Like say, "from date X to Y, civil time will be atomic time slowed by Z parts-per-billion". It just seems like such a better solution than this discontinuity that occurs once every few years and _surely_ everyone would have prepared and tested for. Civil time just doesn't require the accuracy where it actually matters, and if it does matter to you, use a continuous timescale like TAI or compute the current skew offset using a simple formula. Really, I love leap seconds as geeky trivia, but they seem so impractical.

drzaiusapelord• 12 years ago

Whose algorithm do we use? How much time has deviated is arguable and ties in with things like rotation, earthquakes, inertia, tidal forces, etc. Would your proposal be yet another buggy and heavily patched update we need to worry about constantly being updated on the world's IT infrastructure? Incorporating a simple leap second via fiat seems to be a simple and sane solution.

Then what of high accuracy computing? If my clock randomly adds or substracts 2ms a day, that could fuck up my calculations. Better to have things work in a classical sense and to just tack on or remove a second once in a while and be prepared for it in advance. 25 leap seconds have been added since 1972, according to wikipedia. Why make this harder than it needs to be?

Atomic clocks are right and human calendering is arbitrary/wrong. Trying to merge the two automatically can only cause more problems as our arbitrary political and cultural nature could spill over into our scientific nature. See trying to legislate the value of pi in Indiana for an example. The current method works the other way. We push science into our calendering. We keep them nice and separate.

mrgriscom5• 12 years ago

You would continue to use the IERS as the authority on what the true solar time is. They would continue to declare when leap seconds are, but instead of inserting them as a discontinuity, you would simply amortize them slowly over a long period.

Like I said, if you demand high accuracy, use a timescale like TAI where seconds are really SI seconds. But very few people need that kind of accuracy. My computer clock probably already drifts more than 2ms a day. Anyone tracking time for civil purposes is just syncing from a more authoritative source anyway. My computer talks to upstream ntp servers. They could/should hide the fact that leap seconds exist from me entirely. Some of Google's ntp servers already do this.

acomjean• 12 years ago

Calculating the time delta between two events becomes very difficult when your standard unit (seconds) is shifting in size.

that occurs once every few years and _surely_ everyone would have prepared and tested for.

yeah... like y2k, ipv6 and the US conversion to the metric system!

mrgriscom5• 12 years ago

But don't we already have this problem? If, for example, I wanted to calculate exactly how many seconds old I am, I would probably do it the naive way, adding up the years, months, days, hours, etc. But to get the truly exact answer, I'd also have to look up how many leap seconds occurred during my life, making the calculation roughly just as complex. Or I could not bother and just report the result in "civil" seconds, which are slightly and variably longer than "atomic" seconds. If I cared enough about the difference, I probably would be using a true atomic timescale (without leap seconds) anyway.

lmm2• 12 years ago

The second is the physical unit of time; if you tried to skew seconds to be longer or shorter you would break physics (physical constants would change in value, it would be impossible to compare current experiments with previous experiments...).

Would it be smart for civil time to be based on another unit, decoupled from the physical second? Yes, but no-one thought of that earlier, in the same way that no-one bothers to use LVM on the first server they set up. This sort of thing wasn't thought of when governments were first setting up time laws back in 18xx, and many countries' laws included reasonable-sounding provisions like "The average difference between 12:00pm and solar noon shall be zero".

There's a proposal to abolish leap seconds, but it's been pushed back to 2015 to give national governments more time to amend such laws. I suspect and hope that we'll get rid of them sooner or later, but changing one nation's laws is a slow process; changing 190-odd much more so.

mrgriscom5• 12 years ago

That's why I made a distinction between civil time and atomic time. If you're doing stuff with enough precision that differences of parts-per-billion matter, then use SI seconds, which aren't changing. But civil time is for stuff like "what is the current time right now?", a question whose answer hardly ever needs more than 3-4 digits of precision.

Getting rid of leap seconds entirely just seems lazy, kicking the can 600 years down the road with some nebulous "leap hour" concept. Especially so when there is already a timescale without leap seconds: TAI. I'm not surprised this proposal came from the US... "let the next generation deal with it" seems to be our solution to most problems.

MichaelGG• 12 years ago

Leap seconds should be replaced with a leap hour or day or something. Then we could just ignore this forever and hopefully people eons in the future will realise having time sync'd with the sun is ... rather useless.

gpvos7• 12 years ago

Au contraire, it is why civil time exists: to be able to talk about when something happens during the day, and everyone will be confused if 9 AM is not somewhere near the beginning of the work day. Leap hours or days are a cop-out, it's just shifting the problem and making it worse.

klodolph3• 12 years ago

What you are describing already exists, it is called UT1. However, most hardware does not support subtle changes in clock speed.

johnnymonster• 12 years ago

I think more of the internet should be displayed in plain text like this!

stinos• 12 years ago

Except for that indentation (tries to make it look like a letter I guess)

TazeTSchnitzel• 12 years ago

One of the nicer things about RFCs.

bryanh• 12 years ago

A fun, related read on what Google does when they must introduce a leap second to their fleet of servers: http://googleblog.blogspot.com/2011/09/time-technology-and-l...

anon4• 12 years ago

Interesting that they don't keep them in TAI. I would expect having the hw clock of the PC be in TAI to be the most pain-free option. Or is that not possible?

lmm2• 12 years ago

There's very little OS- or application-level support for it. Adding support would probably take more effort than convincing governments to abolish leap seconds.

klodolph3• 12 years ago

This just pushes the problem somewhere else. Smoothing leap seconds gives clock skew of 500ms, requires no changes to software. Handling leap seconds correctly in UTC requires checking millions of lines of code and adding a new way to query the clock. Using TAI causes interoperability problems with other systems; there are 35 seconds between TAI and UTC, and now you have to check millions of lines of code to make sure you correct for leap seconds when you display time to the user. Ugh. Smoothing is the second-best option from a software engineering perspective. Whining about it until IERS stops using leap seconds is the first-best option, the astronomers will have to deal with it.

datenwolf3• 12 years ago

Or, you could just run your computers wall display clock on UT1, which is effectively UTC with the leap seconds "smeared" over half a year. The deviation from the UT1 second to the UTC second is less than the precision most computer's clocks.

cgtyoder• 12 years ago

Do not upset the authorities responsible for the distribution of time.

twic• 12 years ago

Oh, this lot don't just take care of time.

Seriously, check the heading on that memo. You know how people say "money makes the world go round", or sometimes "love makes the world go round"? False. Why do you think they're called the International Earth Rotation Service?

zaroth• 12 years ago

Clearly the proper solution is some space-elevator type device with a rocket on the end to speed up/slow down the rotation as needed and keep our pesky planet turning on schedule!

Would love to see this as a candidate for XKCD 'What If'.

luminiferous• 12 years ago

It's already been done. https://what-if.xkcd.com/26/

joerick• 12 years ago

One thing I've never understood is why we have to shift computer time to match celestial bodies. We have a definition of the second (in terms of a Caesium atom) and we've agreed on the epoch, so why can't we just leave it be?

I understand that if we want to know the time in 'human language' (in terms of the earth's rotation around the sun, and the specific timezone that we're in) we then have to apply these corrections. Surely that's simpler than every timekeeping device on the planet having to adjust a second? Electronic devices are much more likely to be sensitive to a second's difference than us humans are.

gnur• 12 years ago

https://news.ycombinator.com/item?id=7965881 Sums it up pretty well.

a_c_s• 12 years ago

Given the immense cost of leap-seconds, why not just wait for 60 of them to accumulate and add a leap-minute every 60ish years or so?

Being almost a minute behind would have far less impact than all of these unpredictable, tiny changes to UTC.

No leap second will be added tonight

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