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Time is an illusion, Unix time doubly so

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Re: Time is an illusion, Unix time doubly so

#91

Post by masklinn »

hinkley wrote: post #78
masklinn wrote: post #75

Earlier quoted context omitted.

> You expire it at LocalTime + (Expires - Date) Which makes for a wonky mess, and I guess is why Cache-Control did away with the entire thing and just tells you how long the response is fresh.

Not as wonky as you'd think, and 60 minutes means something much different than 12:15. Which can be good and it can be bad, but many of my experiences with TTLs have not been pleasant. 60 minutes, as typically implemented, means some user out there has data that is 1:58 stale and he's yelling at your boss on the phone who is now trying to figure out if he should be passing the favor along. See also ETags which stop t…

> Not as wonky as you'd think, and 60 minutes means something much different than 12:15.

The expression you’ve posted is literally a complicated way of writing 60mn, and you specifically wrote that it should not be interpreted as 12:15.

> See also ETags which stop trying to be clever about dates

Etags have unrelated role and semantics. Etags go with last-update, which doesn’t suffer from anywhere near as much from drift because it’s checked by the server, which emitted it.

Re: Time is an illusion, Unix time doubly so

#92

Post by dwenzek »

photochemsyn wrote: post #64
seanc wrote: post #12

From the GNU coretutils doc [1]: Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done bett…

For even more fun, take a look at this excellent post on GPS, and in particular, the time problem: https://ciechanow.ski/gps/#time > "When it comes to the flow of time on those satellites, there are two important aspects related to Einstein’s theories of relativity. Special relativity states that a fast moving object experiences time dilation – its clocks slow down relative to a stationary observer. The lower the alt…

Thank you for the link! I love this kind of comprehensive blog posts that open a door to a new world. It's worth the time to read those.

Re: Time is an illusion, Unix time doubly so

#93

Post by moring »

photochemsyn wrote: post #64
seanc wrote: post #12

From the GNU coretutils doc [1]: Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done bett…

For even more fun, take a look at this excellent post on GPS, and in particular, the time problem: https://ciechanow.ski/gps/#time > "When it comes to the flow of time on those satellites, there are two important aspects related to Einstein’s theories of relativity. Special relativity states that a fast moving object experiences time dilation – its clocks slow down relative to a stationary observer. The lower the alt…

> Special relativity states that a fast moving object experiences time dilation – its clocks slow down relative to a stationary observer.

For more fun: A less known fact is that SR also states that from the POV of the satellite, the satellite is a stationary observer and the guy on earth is fast moving, therefore that observer's clock slows down relative to the satellite.

Yes, that's right: Each of them sees the other one's clock as slower.

IIRC It takes the fact that the satellite moves in an orbit, not in a straight line, to resolve which one is actually "slower" because up until then, the word "slower" isn't really meaningful at all.

Re: Time is an illusion, Unix time doubly so

#94

Post by mannykannot »

thaumasiotes wrote: post #90
mannykannot wrote: post #89

Earlier quoted context omitted.

That's an interesting point of view, but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law? - IIRC, this is the first-order effect in explaining the analemma. The length of the sidereal day is also variable, but more slowly (at least if we put aside atmospheric and earthquake effects, which I guess affect both equally.) I think we need both a way to reck…

> but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law? I don't see the relevance; the length of the sidereal day isn't constant either. Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds. If days are not of constant length, you could vary the length of a second on a day-to-day basis, or you could define…

> Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds.

Indeed; so when the second was defined by a fraction of a day, that day was an abstraction which approximates any given synodic day. I take your point that the author's quoted statement was incorrect, as the synodic day is a function of the earth's orbital period as well as its rotational one.

I also take your point that we don't care about a specific transition of a cesium atom per se, but I am not so sure that any more than a rather small fraction of the population care that a second is 86400 of what was an average day at some point in time (even though it is is a good approximation for today's days.) What fraction, I wonder, have ever performed a calculation on seconds using that number, other than, perhaps, for pedagogical purposes? Other than that, is it not just a convenient short interval where its constancy (a property that cesium delivers better than does the Earth's motions) is paramount? One might point out that such calculations are frequently done on their behalf, but from that perspective, the recent large increase in the use of GPS, with its heavy computational demand that is dependent on the constancy of atomic clocks, should equally be taken into account.

Re: Time is an illusion, Unix time doubly so

#95

Post by thaumasiotes »

mannykannot wrote: post #94
thaumasiotes wrote: post #90

Earlier quoted context omitted.

> but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law? I don't see the relevance; the length of the sidereal day isn't constant either. Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds. If days are not of constant length, you could vary the length of a second on a day-to-day basis, or you could define…

> Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds. Indeed; so when the second was defined by a fraction of a day, that day was an abstraction which approximates any given synodic day. I take your point that the author's quoted statement was incorrect, as the synodic day is a function of the earth's orbital period as well as its rotational one. I also take your point…

> I am not so sure that any more than a rather small fraction of the population care that a second is 86400 of what was an average day at some point in time (even though it is is a good approximation for today's days.) What fraction, I wonder, have ever performed a calculation on seconds using that number, other than, perhaps, for pedagogical purposes?

It would be unusual to make a calculation directly using the factor of 86,400 between seconds and days.

But people make calculations using the conversion factors of 60 seconds per minute, 60 minutes per hour, and 24 hours per day all the time. If you have those, you can't avoid the conversion factor of 86,400 seconds per day.

Re: Time is an illusion, Unix time doubly so

#96

Post by thaumasiotes »

mannykannot wrote: post #94
thaumasiotes wrote: post #90

Earlier quoted context omitted.

> but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law? I don't see the relevance; the length of the sidereal day isn't constant either. Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds. If days are not of constant length, you could vary the length of a second on a day-to-day basis, or you could define…

> Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds. Indeed; so when the second was defined by a fraction of a day, that day was an abstraction which approximates any given synodic day. I take your point that the author's quoted statement was incorrect, as the synodic day is a function of the earth's orbital period as well as its rotational one. I also take your point…

> so when the second was defined by a fraction of a day, that day was an abstraction which approximates any given synodic day

Commenting separately for this - I'm not sure it's true. To my understanding, the historical European division of the day was that the day contained 12 hours of equal length, and the night contained 12 other hours of equal length. They were obviously familiar with the fact that the day and the night were themselves not equally long, so an hour of day and an hour of night would almost never have been equal durations.

This willingness to vary the length of an hour over the course of a year suggests that it probably wouldn't have been a problem to vary the second along with it.

Re: Time is an illusion, Unix time doubly so

#97

Post by whoville »

cellshade wrote: post #56
whoville wrote: post #49

Earlier quoted context omitted.

> The unix time is synced with UTC No, it isn't. The very article you linked explains that Unix time is monotonically increasing and ignores leap seconds.

"Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number 1483142400 is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00)." From the linked article.

No post body was provided.

Re: Time is an illusion, Unix time doubly so

#98

Post by whoville »

cellshade wrote: post #56
whoville wrote: post #49

Earlier quoted context omitted.

> The unix time is synced with UTC No, it isn't. The very article you linked explains that Unix time is monotonically increasing and ignores leap seconds.

"Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number 1483142400 is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00)." From the linked article.

No post body was provided.

Re: Time is an illusion, Unix time doubly so

#99

Post by 1letterunixname »

Real-time: TAI64 TAI or go home.

Earth time: UT (UT1).

UTC is a vague approximation of UT1 using TAI and a leap seconds data source similar to tzdata.

On most modern CPUs, there is an accurate monotonic counter. The problem though is it doesn't know anything about when it was started or when it went to sleep in any time system.

Oh and bugs. Some systems interpret edge cases in one way, and others in others. Academic purity, interoperability: pick 1.

And then calendars are discontinuous through the years: October 15, 1582 and twice in 1752. They also vary by country which calendar system was in use when. The Julian and Gregorian calendars were in dual use until 1923.

Daylight savings time rules change. Timezones change. And then you get tzdata.

https://www.stjarnhimlen.se/comp/time.html

https://cr.yp.to/proto/utctai.html

Re: Time is an illusion, Unix time doubly so

#100

Post by lproven »

russellbeattie wrote: post #74
lproven wrote: post #41

Earlier quoted context omitted.

There are multiple HHGTTG references in it, as is the author's Twitter bio: > Vell, I'm just zis guy, you know? (A quotation from Gag Halfrunt, Zaphod Beeblebrox's "personal brain care specialist".)

≥ that represents a date around 292 billion years in the future, or roughly 22 times the estimated age of the universe, and thus officially Somebody Else's Problem. And another... (Also, given Adams wrote a few Dr. Who episodes, one which turned into Life, the Universe and Everything, the "Time Lords" reference could be another.) The "Oh, and one more thing" is a slight misquote of, "And another thing." As DNA is my…

> (Also, given Adams wrote a few Dr. Who episodes, one which turned into Life, the Universe and Everything, the "Time Lords" reference could be another.

Nice, a good point, well caught.

> As DNA is my favorite author bar none, I like this post a lot.

Me too.

Plug: join ZZ9. See the wonders of the universe, experience excitement, adventure and really wild things! https://zz9.org/

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