Our perception of time is steeped in the rhythms of our world, and blended with the most ancient of superstitions, decrees and mathematical conveniences. Thus we have days that track our world's rotation relative to the sun (or the sun's apparent motion in the sky), richly divisible (but otherwise unwieldy) 60-based subdivisions of that, weekdays that bear the names of Gods and planets, months based (in the West) on the old Roman standardization of the phases of the Moon, and years that reflect the passing of the seasons on Earth (and thus Earth's orbital cycles around its sun).

Now, in the ages to come, unless it comes to stumble and foolishly fall on its face, Humanity is likely to leave its cradle and spread the gift of consciousness to our solar system, and perhaps other stars as well.

Assuming that a majority of Humans come to live offworld, what is likely to be the most widely agreed upon way of time-keeping?

Keep in mind that:

  • The weight of historical precedent has brought hours from the Sumerians to us, so history has inertia, but the millenia ahead are long indeed.
  • It's likely that humans will one day live so far apart that signals will take days or years to travel from one community to another. Our larger solar system is about 10 of what we now call light hours across.
  • It's possible that humans will live at different timescales due to some humans uploading (and living at Gigahertz speeds?), or due to differences caused by time dilation during long near-luminal voyages.
  • Humans and their machine descendants may or may not retain the hard-coded ~24h sleeping cycle and body rhythms
  • If our computers retain the same basic priciples, we may preserve Unix epoch time for a while.
  • $\begingroup$ I found it interesting that innthe novel Flux, people engineered to, live inside a neutron star used micrometers as a distance unit even though it appled to them differently. Long latency communications is nothing new but a return to normalcy. $\endgroup$ – JDługosz Apr 14 '15 at 2:31
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    $\begingroup$ My humble opinion: since human lived and played on Earth's surface gravity passage of time for everyone is similar, assuming wrap drive is impossible it is difficult to agree on an absolute time. Time dilation due to gravitational force on different locations in space or on any planet as well as when experiencing relativistic travel means everybody's time will be different. I'm optimistic that smart clock can do the necessary compensation so that all times run in sync but how about twin paradox? $\endgroup$ – user6760 Apr 14 '15 at 4:31
  • $\begingroup$ @user6760 warp drive would actually make it worse. $\endgroup$ – imallett Apr 14 '15 at 6:58
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    $\begingroup$ "Day is a vestigial mode of time measurement based on solar cycles. I didn't get you anything." $\endgroup$ – Kyle Strand Apr 14 '15 at 23:56
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    $\begingroup$ Lots of answers on clock time. IMO calendar time is a much more interesting issue. $\endgroup$ – Myles Apr 15 '15 at 22:29

11 Answers 11


Local time would probably dominate. There would be a strong desire to have timescales that line up with local planetary sunrises and sunsets.

However, for universal time, there's a general rule: why invent a new scheme when one already exists? We've been digging away at exacting time references for a while. We may need to update them eventually, but they should stay close:

  • UT1 - tied directly to the earth's rotation
  • UT2 - Like UT1 but with some smoothing factors (historical. no longer used)
  • UTC - Tries to stay in sync with UT1 by adding leap seconds, but otherwise tied to...
  • TAI/EAL - Weighted average of many atomic clocks. Defined to start from an epoch at 1 January 1958 00:00:00 (where it assumed the time from UT2). Originally called TAI, but renamed to EAL so that TAI could be...
  • TAI - Same as EAL, but with an adjustment to account for gravitational time dilation, slowing it down by about a factor of trillionth. This made them act all as though they were at mean sea level.
  • TCB - Barycentric time, which is more effective for interplanetary work. It accounts for gravitational time dilation as though it were a clock at-rest with respect to the solar system, but outside of the sun's gravity well. (Differs from TAI by roughly 490ms a year!)

In theory a more generic time based on galactic rest could be defined. However, this would once again just be a scalar. Most likely we would keep the old Epochs (start of the timer) because there is little reason to change them. Perhaps an interstellar alliance might agree on a compromise Epoch along the way.

The units in the future may not be seconds. However, that is nothing but a unit conversion, so it's easy to add. Its the handling of the atomic clocks and relativity that is so essential.

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    $\begingroup$ Infinity points for listing the universal times we already have. $\endgroup$ – Resonating Apr 14 '15 at 7:13
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    $\begingroup$ @Resonating spawning alternate realities in order to register uncountable sock puppets is against stack exchange policy. Please limit your upvoting to countable infinities in this time stream. $\endgroup$ – Yakk Apr 14 '15 at 13:28
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    $\begingroup$ My bad, I'm sorry. @Cort Ammon I'm taking away nearly all of your points, you only get one point for every positive integer. $\endgroup$ – Resonating Apr 14 '15 at 16:08
  • $\begingroup$ I think the point here is if Earth was suddenly blown up or hit by an asteroid or destroyed, what we now know as Earth-years, UTC time, leap seconds and all that would be meaningless. $\endgroup$ – m1m1k Apr 14 '15 at 17:01
  • $\begingroup$ @m1m1k Yes, though the time-systems which do not add leap seconds would continue to be meaningful. (In fact, there is some talk about no longer adding leap seconds to UTC, because they're starting to think its more trouble than it's worth) $\endgroup$ – Cort Ammon Apr 14 '15 at 17:54

We'll still use the second as a frequency base, defined in terms of physical processes.

To note time in a portable way, like making sure your ship reaches the planet in its orbit, the time in seconds as used by Unix. What reference frame is the time noted in? For purposes of logistics and commerce in the solar system, a suitable reference frame would be devised. It could be much simpler than what is used on Earth now (for GPS etc.) because Earth is rotating. A simple base would be to treat the the sun’s position in space, as measuered by a hypothetical clock that is not rotating or moving with respect to the sun.

But the center of the sun is not really a natural place to measure from. Lets model the sun as a hollow sphere with the mass in the shell and subtract out contributions from the gravity of the planets, and imagine a clock motionless anywhere inside the hollow part.

But that’s getting complicated and the model is limited by observational accuracy and changes over time. Scratch that.

How about looking at distant pulsars. Model their cadence factoring in the orbital mechanics of the system and its rate of slowing. Have several so if one changes due to an anomaly it can be removed and recalibrated. Combine these adjuated frequency bases in a manner similar to what is done with atomic clocks now. Define the time not for the center but for a theoretical distant observer, and that value is adjusted based on your position in the solar system (general relativity) and motion (special relatativity).

Still, the accuracy possible will never be as good as local clocks measuring local time, and technology will drive improvements based on what is possible.

Plotting events in spacetime for data recording will use an single number, counting seconds. But what will humans use? All kinds of things. Mars uses local sols because solar power is tied to the natural rotation. People living somewhere may have cycles that are meaningful to them and those pervade all activity. People generally live in a 24 hour cycle due to biology, regardless of local conditions, but that might be a matter of choice if some simple pill or environmental cues can make normal people cope with (say) the Martian dinural cycle.

Cycles similar to “weeks” appear to be useful, as civilizations have had 5 to 10 day periods for work schedules and personal planning. So whatever can be made to fit with the imposed primary cycles, somewhere in that range, will be a week and the term is deliberately non-standardized and used with local convention.

There is also a desire to sync up with those you’re working with, which is why bases in Antarctica use the time zone of their main supply chain and ignore the correct longitudinal zone. Do astronomers at the south pole (e.g. BICEP experiment), expecially the over-winter caretakers, follow a 24-hour schedule when the sun never rises and the machinery runs non-stop? I’ve never asked.

Computers will have no trouble stamping your log entry with a universal “star date”, and for scientific measurememts it might be needed to indicate your world-line time rate, but that is not needed for human scale events.

If week cycles vary, local custom might name the days creatively with intention to have personal flourishes. Or, a standard may exist for names for days in weeks of each length, or some combination of starting with the common names for work-week but naming days of local significance specially.

To summarize, a colony or worksite may have time cycles of significance, which may be planet rotation period or something completely different. Human timekeeping built around that will be varied, local, and a huge mess. Official timekeeping will be a single scalar number.


While JDługosz may be correct that there will be lots of "local" times, for the vast majority of purposes there will only be the one "universal" time, and most likely a time based on the passage of seconds.

UNIX time is a good measure, by odd coincidence the "starting" time is actually close to when man landed on the Moon (as far as historical events are concerned; it actually started 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970). So if you were to use UNIX time then you could actually date events from their proximity to the Moon landing with a bit of a fudge factor. Centuries from now, most people will probably choose to believe that 00.00.00 really was when man landed on the Moon, much like the AD dates on the Georgian and Julian calendars measure from the "birth of Jesus", or that the new millennium started Jan 01 2000 rather than Jan 01 2001...

Standardizing time this way means that all calendars will match up (everyone knows what happened on 1000197960), and the use of standard seconds will also assist in scheduling, timestamps and electronic record keeping and calculations for spaceflight.

Indeed, the use of local time as opposed to UNIX time will probably be considered an oddity and a way that the particular polity declares they are "not" (or do not wish to be) connected to the grid in any meaningful way. Asteroids and free flying space colonies will have little excuse to not use UNIX time, since their orientation to the home star and period of rotation can be arbitrarily adjusted. In the far future, it may even be possible to generate gravitational torques to cause moons and small planets to rotate at a desired rate, although probably anyone living in that era may simply adjust their own personal environments to meet their needs.


It sounds like you're going for a somewhat hard-science setting, so simple timekeeping is not the only thing you need to worry about.

There are several good arguments already proposed that I agree with regarding the standardization of time (much as we know it today) across the universe and relating it to whatever local cycles exist. For practical and historical reasons it makes sense to just keep the time scale we use today as a sort-of "universal" time, because once you leave Earth it becomes an arbitrary unit, so why re-invent the wheel when we already have everything designed to use this system? (See the time dilation explanation at the bottom for a little more detail on that.)

What is sorely missing so far are the biological implications.

What About the Body?

The human body's processes are based on its own internal clock, what's known as circadian rhythms. Nearly every part of your biology and even psychology are dependent on them. There are numerous studies on the effects of circadian rhythm disruption in the human body that show it to be a slow but ultimately disastrous process. No matter where we go in the universe, our bodies will not operate properly without taking them into account.

Disruptions to these rhythms come from many sources. They seem to be regulated primarily by light, which is kind of a no-brainer since the sun controls pretty much every cycle of life on the planet. Certain wavelengths of light trigger biological processes that tell your body important information like when it should be awake and when it should be asleep, and fighting or disrupting those cycles causes physical and mental stress, insomnia and ultimately leads to higher risk of life-threatening medical conditions.

Deal With It

If you are going to be spreading out, people can't simply "deal with" a different day/night schedule, or even a different wavelength of light from a new star, for starters. They would still need to be on a roughly 24-hour cycle (I think there are some studied which say we operate better on 25-hour cycles but I can't find one) and the light they are exposed to would have to be somewhat regulated to prevent disruption of sleep patterns. You could try forcing people to adapt to a change in that, but you'd then have to allow for 40,000+ years of evolution to take place.

The best way I can think of to handle it, depending on how "hard" the science needs to be, is some kind of minor hand-waving of a genetic modification that tailors people to be resistant to the influence of light, and then they would simply use some sort of sleep-aid device to control their individual biology. I'd recommend not straying from the 24-hour cycle because then you're messing with the core of human biology, but this method would give you enough variation to have people living in perpetual sunlight on a tidally-locked world with a blue star and it not bothering them one bit, and they'd be able to travel to different places with only minor consequences.

Ghost in the Shell

If you have people "upload" their consciousness to a computer, clock time would be critical to their operation (without some creativity on bio-tech or similar that doesn't use traditional computer circuits), but there's no need to deviate from the standard "universal" timekeeping system. If needed for efficiency, these people could develop their own universal standard, but would easily be able to track and convert it to "normal" time almost instantly, so it wouldn't necessarily disrupt anything. What's more interesting to me is exploring how this uploading process would affect their psyche and relationships with their squishier compatriots...

A Final Note on Time Dilation

Time dilation wouldn't really matter for timekeeping beyond finding a formula to keep everyone's watches ticking at the same speed. You'd need a way to sense your relative velocity, and watches would likely have multiple pieces of information on them like how much time has passed for you compared to, say, Earth or whatever universal constant you pick, and perhaps someone's age would be measured by their personal motion instead of in standard years, but that would require constant monitoring of velocity from birth to death. Perhaps people would just learn to estimate it and stop worrying so much about celebrating their age milestones. That could have some interesting effects on society. Legal ages to do various things like drive/fly would be hard to measure, so perhaps they would change to some sort of physical measurement or aptitude test. No matter how you handle it, if you want to explicitly account for time dilation you'll need to read up a bit on it to see exactly how it is measured. I believe Physics.SE has many posts on it.

Hopefully that gives you a good enough overview of the aspects beyond just timekeeping math. Let me know if you need any clarification.


Will they want a universal time?

Universal time is a recent concept which rose to importance with the invention of the train. Before the train, two cities may both have defined "noon" as when the sun was at its highest in the sky. This is a pretty universal definition of noon, but the clocks of the easternmost city were say, 10 minutes ahead of the western city. This was no big deal. Not even that, the citizens would say that if the clocks in both cities showed the same time, then one of the clocks would be incorrect. What's the use of a clock if it does not relate to the clockwork universe and matches the sun?

Today, we have divided the world into 24 time zones for the sake of convenience. Clocks that won't miss a tic in a million years may still be up to half an hour "wrong", depending on their location in the time zone. We have done this because we are now in "simultaneous" communication with people all over the planet. If civilization expands to 10 light hours, this will no longer be the case. In addition, the super-accurate clocks people carry along will become out of sync due to the effects of relativity.

Maybe our priorities in time-keeping is the exception? A curiosity in-between the pre-industrial and the spacefaring time periods, when "simultaneous" and "synchronized" was both real and important concepts.

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    $\begingroup$ I really like that last paragraph. It opens up many interesting ways society could evolve if it became less-focused on those concepts. $\endgroup$ – thanby Apr 14 '15 at 13:40
  • $\begingroup$ Well, if you have a date with your beloved in space and you use the wrong time you'll likely end up not only alone, but a few a million km away from your sweetheart. $\endgroup$ – Serban Tanasa Apr 14 '15 at 17:20
  • $\begingroup$ Think about how society would adapt to such things for inspiration :) Look back to Medieval-era communications, where one part of a family might be away for years with little or no contact to the rest and only a vague return date, barely accurate to a season if even a year. Farewells and reunions were much more internally intense due to their unpredictable nature, but were such a normal part of life that people were forced to handle them gracefully. Without the possibility of FTL we'd be left in a very similar situation across the stars. $\endgroup$ – thanby Apr 14 '15 at 20:57

Each planet would have a standard time, with periods that make sense. Mars day is 20 some minutes longer than Earth. So you could just add a couple minutes here or there, or do a time slip, where the clock his midnight and stops for 20 minutes, then starts again.

Each planet would have its own time, and then there would be a galactic time, like the Unix timestamp or Star Trek's stardate. Something that has no relation to any planet or star. It would mostly only be used on board starships and inner inter planetary communications/business.

Maybe base it on rotations of a central pulsar or something. Something dependable.

Humans lives are dictated by circadian rhythms, meaning we sleep, then wake, eat, and eventually have to sleep again. Some people have tried to get around this, but none of the techniques are all that effective. Living outside of the rhythm (3rd shift workers for instance) is hard on your body. You can do it, and get used to it, but 3 A.M. is still going to be a low point.
If they discover a cure for sleep, this could change.

On different worlds with different stars and lengths of day the rhythm will have to be adjusted, but something that feels natural will work out, to the point that most people sleep during one period, and are awake during another period, and eat at roughly the same times, and from that a local time will develop naturally.

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    $\begingroup$ Wouldn't inter planetary communication make more sense for using a galactic time, instead of inner planetary communications? $\endgroup$ – DrCopyPaste Apr 14 '15 at 10:07
  • $\begingroup$ I question this. Local time works well for planets like Earth... But the majority of planets are not like earth! What is the use of a day that lasts 5 minutes, or 500 years? Or infinity years for a tidally locked planet? $\endgroup$ – Fhnuzoag Apr 14 '15 at 13:11
  • $\begingroup$ @DrCopyPaste Yes. I typed it on mobile, so it auto corrected. That is my story, and I'm sticking to it! $\endgroup$ – AndyD273 Apr 14 '15 at 13:41
  • $\begingroup$ @Fhnuzoag In extreme situations galactic time could work on a planet, but I'd be willing to bet that any planet with close to normal cycles will develop a local time of some type. It's just useful to say "I'll be there around noon", which is when the local sun is straight over head, vs "I'll be there around Stardate 92889.71", and then you have to pull out your computer and figure out how long you have till your ride gets there. Even if they get away from a "normal" 24 hour clock to something decimal, it'll be customized to local time, so 50.00 is when the local star is straight over head. $\endgroup$ – AndyD273 Apr 14 '15 at 13:52
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    $\begingroup$ Time is an illusion. Lunchtime doubly so. Nevertheless, I agree that the people will structure their local time around the typical sleep/wake cycle. One only needs to worry about disparate clocks when actually interacting with other places, and at that point I would expect computers to handle any translation issues rather transparently. $\endgroup$ – pluckedkiwi Sep 18 '15 at 19:33

Observing a predetermined pulsar from all locations could be used to measure time. Since everyone in this scenario will be of earth origin and the frequency of the predetermined pulsar is known as observed from earth, all local time measures can be converted to an earth measurement or any other civilization that has made sufficient observations of the pulsar at their location to account for any time dilation.

Communications could be timed stamped at the current pulse count and future dates based on the count.



Assuming that a majority of Humans come to live offworld, what is likely to be the most widely agreed upon way of time-keeping?

Over the eons, measurement/calculation of the Planck length would give us a smallest unit of time, we would almost certainly redefine our second as a multiple of this smallest unit.

We would use seconds as a universal standard and an agreed epoch to start counting from. At the moment this is the Unix Epoch. Given rising computer word sizes as well as storage space, this would probably eventually be adjusted to an absolute and invariable reference such as the birth of the universe.

Universal Time would probably be separate from Local Time and use powers of the base to track passing of time. At the moment the base we use is ten, it could possibly be different in the future.

Local Time would reference the nearby cycles and scale up in cycle length and importance. Local Time would be used everyday as significance could be attached to a particular "hour", etc. Universal Time may be the Local Time if there are no cycles of importance (they must have regular cyclic importance) this seems more likely in "uploaded" individuals, especially if they have no internal cycles.

For the uploaded individuals they may move towards using the smallest unit of time as a "second". Mostly because the second's status as a base unit is because the time-scales and reference point are relevant to us. If we get to the point of fitting a whole day's thought into the space of a second we would necessarily redefine the "second" possibly adopting a new name so we could keep the old one. The necessity is best seen when looking at schedules. Looking at an itinerary "Call Home @ 5:30" doesn't make sense if 1/24 of a second feels like an hour, you would necessarily redefine the hour to 1/24 of a present day second, or perhaps adopt entirely new measurements.

On a closing note its probably best to observe that our units have changed over time. While not much because of historical inertia, when you leave history behind and visit the stars you lose that inertia holding you back.

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    $\begingroup$ Planck time is definitely the way to go! I don't see how anybody could not like "see you in 1E49 tp's" ;) $\endgroup$ – Ghanima Apr 14 '15 at 17:56

It is very likely that future civilizations will keep different time measurement scales for different events in spacetime.

For religious ceremonies each religion would need to decide if the ceremonies are to be performed as per local time of the person, or as per the time of some specific location such as Earth. Note that we already have discrepancies here on Earth now, for example the Muslim year is shorter than the Gregorian year by I think 12 days. Thus, when a Muslim tells you that he is 65 years old, he may have been born only 63 years ago by the Christian way of counting years.

The marking of anniversaries such as independence days, birthdays, remembrance ceremonies, and marriages would likely be done by the local time for the places or the people. Titan may celebrate 2 independence days for every 120 times that India celebrates its independence. However, an Indian on a spaceship to Alpha Centari and back may need to celebrate 15 Indian independence days during his 700 day voyage due to time dilation. During that time, his parents only celebrated their child's birthday twice for the 15 years they were waiting for his return.

Legal privileges and responsibilities such as drinking age and driving age will need to be coordinated with the time dilation experienced by the person. Perhaps national ID cards will have a timekeeping device, similar to how radiation workers wear dosimeters today.

Activities that must be performed on a schedule will need their time computed as per the location where that activity takes place. The seasons of a planet do not change just because the farmers travel to other star systems. The planning to return to some planet in order to tend to the crops in season there will have to be coordinated to happen once each time the planet revolves around its sun.

I imagine that we will need computers to help us calculate when the events take place, and when we must leave our current destination in order to get to the proper location at the proper time, taking both distance and expected travel velocity into account.


I came up with this idea for "universal pulsar time" a while ago, while struggling to write scheduling software that can handle timezones and leap ... things. Everyone synchronizes to one pulsar. Its period is the new second.

Depending on your relative speed and the gravitational field you are in, you may perceive fast seconds or slow seconds. Over time, the pulsar will slow, and the second will get longer.

However, whenever any two or more people rendezvouses, their dumb clocks (pulse counters) will agree that the same number of seconds has elapsed since they last met. I believe this is the most important property of a timekeeping system.

  • $\begingroup$ unless you're trying to use the seconds to time your rocket burn for your descent to visit the friend... $\endgroup$ – Serban Tanasa Apr 15 '15 at 16:26
  • $\begingroup$ Aw crap. Well maybe instead of messing with the clock, we should integrate relativistic time dilation and the pulsar's angular speed into all of our time based equations. But then we lose simplicity, which was the whole point of synchronizing on a pulsar... $\endgroup$ – Dan Ross Apr 15 '15 at 16:28

Warhammer 40K has already come up with a dating system: Imperial Dating System.

This is based on the idea that the only authoritative source is the Sol system, and that each other system will have their own dates, which may advance at a faster or slower pace than sol does. Their solution, is to make an accuracy marker part of the date.

The key point, is that you need two things: 1) an authoritative source, and 2) a way of syncing with that source. You could change the imperial dating method so the accuracy check digit refers to how long it has been since the last sync, but having some way to doing the correction is essential.

Since your civilisation is galactic, they must either have some FTL method of communication already available to them, otherwise you might as well just have each region use their own local time - the only reason you would need a coordinated universal time, is if you need to coordinate events between time zones.


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