In "Deepness in the Sky" a spaceship's computer is still running on Unix time many thousands years into the future. It is generally believed that the calendar starts with the first Moon landing, and only the main character as a "programmer-archeologist" discovers a small difference between it and true Unix time 0.

Little is known about “Unix” but it is believed he or she was a charismatic spiritual leader who brought hope, healing, and comfort to countless humans on Earth and inspired numerous disciples including “BSD”, “Linux”, and “System V”.

> Little is known about “Unix” but […]

As opposed to eunuchs.

Well, isn't Unix a castrated Multics?

Vernor Vinge sounds like an intriguing author!

“Most years, since its inception in 1999, Vinge has been on the Free Software Foundation's selection committee for their Award for the Advancement of Free Software.”[0]

Since “A Deepness in the Sky” was written in 1999 as a prequel to “A Fire Upon the Deep” (from 1992), which do you recommend to read first?

[0] https://en.m.wikipedia.org/wiki/Vernor_Vinge

Fire, then Deepness, then stop.

Or Fire, then The Peace War. Maybe dip into True Names, followed by Shockwave Rider by John Brunner.

Many people didn't like Deepness much. But the sequel to Fire, Children of the Sky, was interesting.

For me A Deepness in the Sky is his best, and one of the most memorable novels I've read.

The Peace War is a fascinating exploration of "what if this one thing was possible?" and all its terrible consequences, but Deepness grabbed me much more as a story.

In either case, definitely read The Shockwave Rider by Brunner.

Personally I found Fire and Deepness to be equally, profoundly, good. Haven’t found any other Vinge books to be nearly as good.

I quite enjoyed Rainbow's End as well.

There's an extra apostrophe there... this is addressed toward the end of the novel.

Yes, it's not "The end of this particular rainbow", it's "rainbows have an end". It bugged me until I got to that part of the book.

That comment is a bit of a spoiler, isn't it?

"A Deepness..." also used SI multiples of seconds to denote something approximating 24 hours, a week, a month etc.

A megasecond is a useful quantity: it's a bit less than 12 days. I mentally approximate it to a fortnight, partly because of the VMS bootloader which used microfortnights as an approximation for seconds https://en.wikipedia.org/wiki/List_of_unusual_units_of_measu...

Another handy time_t approximation is that a year is about 2^25 seconds, or 32 megaseconds.

He should have said what that difference is.

There is a very good reason to eventually abandon it: leap seconds. Unix time goes back one second whenever there is a leap second on Earth.

It's extremely weird and IMHO completely ruins the purpose of a timestamp, but it's a compromise for backwards compatibility, since Unix time was created before leap seconds. This hack ensures that the number of seconds in a day remains fixed, an assumption of many systems at the time.

Leap seconds were introduced in 1972. The time_t that you are thinking of was invented somewhere around 1974, according to Unix historians.

Ironically, your kernel measures TAI time perfectly happily if unmolested, as that always ticks once for every SI second (ignoring oscillator drift). No slewing. No smearing. No stepping.

Isn't a second just how long light takes to travel 299,792,458 m?

Also, "since Unix time was created before leap seconds." I found this interesting for the simple reason I've never spent any time thinking about Epoch vs Leap Second histories.

Yes, but the point is that the UNIX timestamp doesn't count the the number of seconds elapsed since January 1, 1970. It counts 86400 seconds per day, regardless of how many seconds the day actually has (which can vary due to leap seconds).

Is this accurate though? Isn’t the number of seconds since 1970 absolute, and it’s up to the library generating the Gregorian date to take into account leap seconds? I suppose if these libraries are not taking into account leap seconds, then the actual rollover will be a few seconds earlier (or later?) than what we think.

See the definition of seconds since the epoch at https://pubs.opengroup.org/onlinepubs/9699919799.2018edition...

  tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
      (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -
      ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400

Your time_t isn't the true number of SI seconds since the Epoch. It's that number, mucked around 27 times so far. Instead of leaving their kernels to count one tick per SI second (ignoring oscillator drift) many people slew, step, or smear their seconds count every time that there is a leap second.

Go and read about "right" versus "posix" timezones.

* https://unix.stackexchange.com/a/294715/5132

IIUC unix timestamp simply counts seconds (well defined interval of time) from epoch onwards. It is then up to different calendars to interpret this number as a given day / hour /... This is the beauty of it - it is always an accurate (well, at low speeds at least ;) ) measure of time. In other words, no, some days might not last 86400 seconds.

This is not correct. UNIX timestamps always have 86400 seconds per day, and consequently, they do not actually count the number of seconds since the epoch.

I stand corrected - thank you for the clarification!

And meter itself is defined as the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

So it's cyclic and doesn't make sense outside the earth reference frame

Edit: since 2019, 1 second is defined by taking the fixed numerical value of the caesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1

So a second makes sense in the galactic scale at least as long as Caesium-133 is a stable isotope in that environment

> Since 1967, the second has been defined as exactly "the duration of 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

> realisation of the metre is usually delineated (not defined) today in labs as 1579800.762042(33) wavelengths of helium-neon laser light in a vacuum

No. A second is "The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom."

No, that's how the metre is defined. The second is defined by the transition between two hyperfine states of cesium 133 (9,192,631,770 'oscillations').

Or lets say we colonize a planet with a significant time dilation (ie near a supermassive blackhole), how do you deal with this? Who has the canonical time?

AFAIK, there is no canonical time in relativity. There is the "local time", which is what your wristwatch shows, and the "time in place X", where X can be Earth, Mars, or some spaceship. These times might not be in sync, and they might even be distorted. Because, if place X is moving relative to you at a significant percentage of lightspeed, their seconds will be longer. Also, the further X is, the blurrier the concept of simultaneity becomes. Which makes the question "what time is it _now_ in place X" moot :)

If relativity don’t allow time travel doesn’t it make time canonical? A clock on Earth from telescope might look jumpy but only in forward direction after compensating for distance I think

Looking at a clock from Earth through a telescope isn't as straightforward as you'd think. The image you see of that clock is actually light (photons) emitted from Earth, which will take a while to reach you - like, 1 year, if you're 1 lightyear away. During that year, Earth has moved on, maybe blew itself apart. But you can't even tell, because information can't reach you faster than light :) So you can only see Earth's past, not Earth's "now". The further you are, the more "now" loses meaning.

Yeah but can it ever go back, other than by backing off faster than the speed of light? If it can’t then it’s at least monotonic, even if it’s not rate constant

All observers observer all clocks to advance monotonically forward in time, regardless of their location or relative speed, unless the clock is moving at the speed of light. But the rate that each clock is observed to advance at depends on things like relative speed, acceleration, and the curvature of spacetime.

... unless it's the system clock in Linux, with the hardware RTC set to be interpreted as local time, and one is watching a system boot on a machine that is set up as east of the Prime Meridian (i.e. the hardware RTC is ahead of UTC). (-:

* https://unix.stackexchange.com/a/600490/5132

Given that Unix time specifically doesn’t handle leap seconds, a system clock is not a clock by the conventional definition.

We'll probably come up with something similar to time zones once we become an interplanetary species.

"Local seconds since 1970", vs. "Static seconds since 1970".

You don't have to go to science fiction for this. Science fact will do. On Earth, with Atomic Time, we already have to account for general relativistic effects. Doco from the BIPM (the SI Brochure) already talks about accounting for them across the sizes of the measuring devices.

You'd just use some math to remove the influence of the black hole. Or for a more hands-on answer, you could have some reference beacons that are significantly further away from the black hole.

In a broad sense, the answer goes like this: Almost everything in the galaxy shares an approximate reference frame when it comes to velocity and special relativity. For general relativity and gravity, that's obviously a distortion so it doesn't count.

In the long term, to prevent very minor drift, we can use quasars as a reference.

Why would someone on Mars, or an asteroid, or orbiting venus, care about leap seconds on earth?

Number of seconds since a given time seems fine until we're travelling significantly outside of a common reference plane.

That’s exactly why op said we should abandon Unix time. The “who cares about leap seconds” standards is called TAI, and the difference between TAI and Unix time changes every time a leap second is introduced.

Unix time decrees every day has exactly 86400 seconds. But the introduction of leap seconds means that While most days have 86400, some earth days have 86401 seconds and potentially there could also be days with 86399

TAI makes sense everywhere, Unix time only on earth.

Because that's not how Unix time is defined. The leap second hack is hardcoded into it:

Unix time is [...] the number of seconds that have elapsed since the Unix epoch, minus leap seconds.

- Wikipedia

If you are on Mars you'll have to update your computer time every ~6 months when Earth release a new table of leap seconds caused by e.g. earthquakes.

Or we just pretend they never happened.

The galactic GPS would be based quasars, e.g. far away galaxies, that would pretty much appear the same positions anywhere in the Milky Way. Quasar spins are gradually slowing down. So you could tell how many light years you are from the quasar by the observed spin rates. In addition, you can determine epoch time from knowing position a spin decay rates.

Carl Sagan put a quasar GPS on a golden plaque on both Voyager probes, so future intelligences could tell where and when the Voyagers came from.

Next iteration of unix-time should include the concept of local time in context of relativty if we are going to travel or communicate over significant distances.

How? And what would be the place/value measured against? And would all "timekeepers" need to get signals from that place to get their offset? If so would we need to take lensing from gravitation into account?

How do you even measure a global value when the local perspective by definition changes the value?

Einstein: There is no such thing as an absolute time

Programmer: Hold my beer

More like:

Programmer: Next iteration of unix-time should include the concept of local time in context of relativity if we are going to travel or communicate over significant distances.

Einstein: Hold my beer

You mean that from a remote place or fast-moving ship, the time on Earth would appear to go at a different speed? So local time would not just have a constant offset?

If time is moving at a different speed, it wouldn't just be a constant offset. You'd also need a multiplier. Think of two cars moving at different speeds on a road. They don't stay the same distance apart.

Yea, but if we're going to use a "number of seconds since ..." Type system like Unix time, SOMEONE has to be the origin point, so I don't think this is a knock against Unix time.

Its unfortunate to use an accelerating reference (earth rotating sun, sun rotating galaxy, galaxy intermixing with andromeda) because it requires you to awkwardly keep track of the astronomy of earth for timekeeping. Instead, you would want a standardized reference time frame, say, "unix time assuming earth in 1970 never experienced any acceleratiom or dilation" and then everyone, including earth people, would track their acceleration and dilation offsets to be able to compare time when meeting or convert signal timestamps from any source back to their local time.

Well we already have to keep track of the size and shape of Earth. Go and read the SI Brochure from the BIPM. Local measurements by atomic clocks on Earth have to be weighted according to how far they are from a reference surface of equal gravitational potential. There's a publication named Circular T that comes out every 5 days.

You don't need science fiction for this.

* https://www.bipm.org/en/bipm-services/timescales/time-ftp/Ci...

You would want some global reference time, and that could be a number of seconds passing on Earth. However locally you can't work with a clock that doesn't tick at a constant rate (if seconds on your spaceship go twice as fast, the crew can work with that; if the rate changes day-to-day as you accelerate, that's a problem).

In addition, you will probably want clocks that run at a given perceived speed, to time physical processes that take the same time everywhere (e.g. so your cookie recipe saying "bake for 600 seconds" work without translation).

Quasar time plus position, motion vector, and acceleration vector should be a good enough reference for anybody.

You can't do math on that directly though. You'd still need to convert them to a common reference frame for comparison.

With Earth's day length becoming irrelevant, the 24 hour clock becomes irrelevant too, and the length of the sleep cycle may shift.

We could eventually arrive at stardates with one starday being 100000 seconds. Of course, that would be just as arbitrary as simply keeping the 24 hours of 60 minutes each.

Humans have a remarkable circadian rhythm that does a pretty good job at defining a day as “24 hours.”

There were experiments with forced desynchronization of human circadian rhythms.

http://www.chronobiology.ch/wp-content/uploads/publications/...

They were very limited, though. We do not really know what would happen on a spaceship that changed its day to 100 000 seconds and subjected the crew to this cycle for months or years.

Given how adaptable people usually are to external influences, I would guess that some adaptation would take place.

To my knowledge, once you take the sun away, it tends to drift more towards 25 or 26 hours. And that's in humans who experienced the externally imposed 24 hour cycle for decades before participating in a short experiment, not potential spacefarers born onboard of stations or ships.

Wont there be issues using the same time on different planets since time speed is not constant?

Or some other significant event in the eye of this civilization.