In my next novel, there are three major powers in the solar system: Earth, Mars, and Saturn. Earth runs on a slightly modified Gregorian calendar, Mars goes with the good old Darian calendar, and Saturn... well, I'm really not sure what to do with Saturn.

The Saturnian calendar must:

  • be cyclical; while it doesn't have to use years per se, it must have some similar sort of cycle by which to measure large spans of time.
  • Use Saturn, Jupiter, and Uranus as reference points.
  • Have days. Humans still sleep.
  • Not be an endless count up like Unix time or Mars mission sol counts.
  • Tick at humanly relatable intervals. If each cycle is defined as a Uranian year, that's just ridiculous.

Other important background information:

  • The people of Saturn live on the moons thereof, as well as in various satellites. No wonky gas giant creatures.
  • The nation of Saturn holds territory on the moons of Jupiter, as well as a monopoly on territory around Uranus. Uranus is essential to their continued survival as a nation, because the Helium-3 that powers their ships comes from there.
  • The Saturnian calendar was designed recently and from scratch, so it's not quite as prone to historical weirdness as the Gregorian one.

What sort of calendar would humans settling the moons of the inner three gas giants use?

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    $\begingroup$ I'm voting as Primarily Opinion Based for now, not to denunciate you or your question, but because any answer is equally good right now. Please try thinking of criteria for your calendar system, what is good/bad, what does the calendar need to accomplish/fulfil? $\endgroup$
    – dot_Sp0T
    May 6, 2019 at 5:12
  • $\begingroup$ To elaborate, the following brief answers are right now, equally good: "Humans would use a gregorian calendar, because they are used to it"; "Humans would use a calendar based on the orbital period of the main body, because that's a 'year'" $\endgroup$
    – dot_Sp0T
    May 6, 2019 at 5:14
  • $\begingroup$ @dot_Sp0T I added some criteria to clarify. Do you think those are enough? $\endgroup$ May 6, 2019 at 5:23
  • $\begingroup$ I think your edits make this a very good question, yes :) btw you don't have to put any edit notes in the question body, the editing system on the site keeps track of all edits and allows rolling them back, etc - VOTE RETRACTED $\endgroup$
    – dot_Sp0T
    May 6, 2019 at 5:25
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    $\begingroup$ I'm voting as POB simply because any definition of time is arbitrary. Calendars do have some aspects that are objective (lunar/solar cycles), but when it comes right down to it, seconds, minutes, hours, days, and weeks are 100% arbitrary. You don't say where the residents of Saturn came from. Earth? Why wouldn't they use seconds (the basis of human mathematics), and therefore everything else? There's half a good question here, but somehow we need to get away from the arbitrary definition of time to avoid being POB. $\endgroup$
    – JBH
    May 6, 2019 at 7:45

5 Answers 5


Saturn rotates around its own axis every 10 hours and 33 minutes (or it does at the top of the cloud layer; there is no solid surface to speak of). People living there may have adapted to sleep every third day, having three-day cycles of sleep, work, and leisure (possibly in shifts). Such a three-day period could serve as a "Saturn day" of 31 hours and 39 minutes.

Saturn's largest moon, Titan, has an orbital period of 16 Earth days, or 12.09 "Saturn days". This could serve as a Saturn month, with months of 12 days with occasional 'leap months' being one day longer.

Saturn's solar year is 29.46 Earth years or 8,163.7 Saturn days. It may be a bit long to serve as a calendar year, but may mark generational cycles.

An alternative is using Saturn's third-largest moon Iapetus' orbital period of 79.32 days as a year. This corresponds very well to five "Saturn months" (4.973, to be more precise) and to 60.15 "Saturn days".

This creates a calendar with quite close to 12 days to a month and 5 months or 60 days to a year. There would be 4.6 Saturn years to an Earth year. Adulthood may start at 80 Saturn years (17.4 Earth years).

  • $\begingroup$ Should 1 Saturn year not be the time it takes to go around the sun once? $\endgroup$
    – Mixxiphoid
    May 6, 2019 at 11:39
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    $\begingroup$ @Mixxiphoid OP required that the calendar should Tick at humanly relatable intervals. $\endgroup$
    – J...
    May 6, 2019 at 12:23
  • $\begingroup$ @J... ah, missed that last point! $\endgroup$
    – Mixxiphoid
    May 6, 2019 at 12:42
  • $\begingroup$ This "Saturn month" is close to a fortnight in length. $\endgroup$
    – Jasper
    May 7, 2019 at 5:13

On Earth calendars serve two basic functions :

  • Agricultural and resource management - observing yearly cycles.
  • Administrative
  • Standardization

Administrative requirements just need any calendar - it's completely arbitrary when it starts or stops. The seasons do not matter. It's simply a day counting thing so we can e.g. set tax years, prison sentences, contract schedules, travel schedules.

Standardization is very important. It means everyone uses the same basis for their calendars and avoids problems in organizing everything in a complex world. Standardization can also use an arbitrary calendar, just so long as everyone uses the same one.

Which leaves us agriculture and food production. This is really why we need calendars and why they were developed. Administrators and standardization got bolted on later as useful, but the original purpose was to make sure we plant crops, harvest and so on at the right times. It was a disaster to get these wrong.

But in space colonies we're not going to be need to observe local seasons as they have no human purpose. The computers can track any important events better than we can and let us know, so we can ignore the only reason to lock a calendar to a local astronomical or planetary cycle.

We already have a very highly developed "home" calendar on Earth and we're going to export that. We'll define some arbitrary standard, using our computers to do the maths and our atomic clocks to do the ticking. Keeping things synchronized is more complex in the very long run, but that just means standard's committees invents and using leap-years, leap-days, leap-hours, leap-minutes and leap-seconds (which they already do, so it just keeps them employed :-) ).

The Saturnian calendar must:

"Must" ? Probably not.

be cyclical; while it doesn't have to use years per se, it must have some similar sort of cycle by which to measure large spans of time.

We'll use what we have already. It works.

Use Saturn, Jupiter, and Uranus as reference points.

Totally pointless, actually worse than pointless as it's useless to the humans involved. There's just no function to it, but it would create utterly pointless complications in dealing with other planets and moons and space stations.

The weather doesn't matter as we can't live in the local conditions without suits and ships and vehicles and completely enclosed buildings. The local day or night doesn't matter because humans have a very hard-wired biological clock and trying to work against this clock will damage the health (mental and physical) of a human very quickly. The Earth's day-night cycle is not optional.

So we'll be completely ignoring local basis for the calendar.

Have days. Humans still sleep.

Precisely : and they do it to a hard-wired Earth day-night cycle. I'd suggest trying not to work to anything else, except it will rapidly screw your health up, so don't even perform the experiment.

Not be an endless count up like Unix time or Mars mission sol counts.

So we use those well established human things : days, months, years, centuries. Don't fix if it ain't broken.

Incidentally almost everyone in the industrial world relies on their smart phone to tell them the date, time, etc. and that's using a Unix time and date system. It's here to stay.

Tick at humanly relatable intervals. If each cycle is defined as a Uranian year, that's just ridiculous.

So what's the problem again ? We use Earth-related calendars because that's what will work for the humans.

Oh, yeah, you said this :

Mars goes with the good old Darian calendar

No it doesn't. It's that simple.

For all the reasons I've stated (and more) there is no way an organized, financially connected, human populated system will be adopting an artificial calendar like the rather academic Darian.

In the real world Mars will be populated by people who have ongoing business with Earth, operate to a standard Earth calendar and in the majority won't care a fiddler's damn about the obscurities of Martian orbits or "weather". It's only weather when you see it and practically every "Martian" will be living in a world locked off and out of direct contact with the real Mars (or be dead).

So they'll standardize on what they need day-to-day, which is Earth's standard calendar for business and finance.

Forget the Darian calendar - it's drivel that keeps a very small number of people amused, but there's no practical reason for a real Mars colony to use it.

I can tell you one thing human history shows us : regardless of what army winds and how long an Empire lasts, the bloody standard's committee seems to survive forever. The Roman empire may be long gong, but their standards committee seems still to be alive and well and working in ISO-land. Likewise the British Empire and the French Empire. Standards have nothing at all to do with reality, but with the practicalities of business.

There is no useful purpose to the organizers and standard makers who need to sync with an Earth calendar in using a Darian calendar. It's not going to be anything other than an obscure concept if there ever is a real Martian colony.

Even if scientist and engineers started with Darian for some geek-crazed reason, the first wave of non-scientific settlers would rapidly say "what the heck for ?" and use what they know and relate to - Earth's calendar (which their relatives, friends, business links and politicians on Earth will be using).

The Saturnian calendar was designed recently and from scratch, so it's not quite as prone to historical weirdness as the Gregorian one.

This is not going to be possible. Planets may, on some superficial level, seem regular, but even Earth's calendar has ended up the way it does because there is no natural "all-integer" way to build an Earth calendar that wouldn't be rejected as being too different from (yes) The Standard.

So you're on different Moons of Saturn. What difference or use is a locally based calendar to you ? None. Just like Mars you need to exist in a human friendly bubble to survive. Saturn's cycles have no meaning for you, the ordinary dweller. You work e.g. in a a bank on a moon of Saturn. What's more important : the date on Earth (observed by all its financial markets) or the local Saturnian date ?

Long before your colonies become powerful enough to impose their own standards on Earth, the imported standards will be so established and have such a locked in inertia that no amount of flag waving would shift them. Again - the Roman Empire fell, but its calendar did not !

Even if Earth was destroyed, its main calendars would live on. With the standards committees.

  • $\begingroup$ Downvoters : If you don't say what you don't like about an answer (or question for that matter) no one, including the person who wrote the material, can ever know what the problem you think is there is. $\endgroup$ May 6, 2019 at 15:02
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    $\begingroup$ Yes. Compelling answer. There wouldn't be one unless there was a compelling reason to have one. The ISS has 90 minute long "days", but they do not have a special calendar. I would expect the local daylight hours to still matter to the colonists however, and feel this was a bit brushed over in this answer, but it's possible that they wouldn't be able want to sleep to them and so would just keep track of them in terms of earth time. $\endgroup$ May 6, 2019 at 15:24
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    $\begingroup$ I think the asker got what they wanted with the selected answer, but this is definitely the most likely answer and a good partial frame challenge. Colonists might have a local day/night cycle displayed alongside standard time but the Earth calendar will still be the unified time system. $\endgroup$
    – ben
    May 6, 2019 at 20:37
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    $\begingroup$ You're right about Saturn, but not about Mars. The notion of a "day" being the time it takes for the Sun to return to the same position in the sky is meaningful. On Mars, that's not quite 40 minutes longer than it is on Earth, so people living on Mars will probably track both Mars Standard Time and UTC (or a derivative, possibly representing the time a radio signal from Earth emitted at that time reaches the Martian capital) for dealings with Earth. The various satellites of Jupiter and Saturn have rotation periods that are too far from human norms to be valuable, so they'll just use UTC. $\endgroup$ May 6, 2019 at 21:41

This might not fit your setting well as you want some relation to the outer planets, but having come across a similar issue while building my world I thought presenting my solution could be helpfull.

This calendar is used for ship and station time and on worlds where the rotational and orbital cycle, aka day and year are irrelevant or impractical. Should your Saturian culture consider itself to be very progressive and scientifically advanced this calendar would be very fitting.

The basic idea is that it would be great if there was a standardized unit of time widely used in science and exactly defined. One that could be used without crazy conversion calculations and disputes over its functionality and correctness.

Wait there is such a unit! The second

Here is a timetable giving the units and their most closely related Terran equivalents. The names are based on standard unit prefixes or derived from them for better usability. These are template names. I chose the ones best fitting the culture I'm working on. Thus I suggest you do the same.

Nova: 1s = 1s

Deka: 10s =10s

Hekto: 100s = 100s (probably replaces the old minute)

Kilo: 1.000s = 16,7 min (a quarter of an hour is already a commonly used unit of time)

Dilo (DekaKilo): 10.000s = 166,7 min (new hour, i suspect that half Dilos would be popular for everyday business)

Hilo (HektoKilo): 100.000s = 27,264 h (the new day, just 3 h 16 min longer, which is not too bad)

Mega: 1.000.000s = 11,35 d

Dega: 10.000.000s = 16,53 weeks

Hega: 100.000.000s = 3,17 a (new year and base number for yearly time units)

Giga: = 32,49 a

Diga: = 321,5 a


The great thing about this is that many of the new time units already have some close equivalent in our timetable.

For us this way of measuring time is mildly confusing, but it is really convenient to use once one gets used to it. I suspect that not all units will be used equally, for example the Dekasecond seems slightly useless.

While one could run this system like an endless count, the same could be done with our Earth system. We just don't do it. I suspect the same will be done with this system. Ignoring the Nova and Deka digits seems reasonable and the Hilo (day) cycle mathematically resets each Hilo (day). Use this example to work out the proper digit combinations for other occasions. In the end it is a rounding game, so having an appointment at 6 Dilo or Dilo 6 (sixth hour of the day) sounds fine.

  • $\begingroup$ Why the spelling "secound" instead of just using "second" if the units are identical in length? $\endgroup$
    – Hearth
    May 7, 2019 at 1:00
  • $\begingroup$ @Hearth It's a typo, I'll fix it soon. $\endgroup$ May 7, 2019 at 4:03

StephenG is generally right, Earth calendar is probably best choice. But I'll structure this answer bit differently and from a different point of view. Think of this as a supplement?

First, the day. Human diurnal cycle evolved on Earth. So our circadian clock defaults to so close to 24 hours that it would be silly to use anything else. Martian day is close enough that Martian colonies can use that instead but in your case the Earth day should be the basis. Or possibly the Martian version or some sort of unified day that is a compromise between Earth and Mars. But that is politics.

When you have the day that is the same as on Earth there really is no reason to use units of time that are different from Earth standard. So hours, minutes and seconds should stay the same. If you want to be different the seconds are the ones that would be an issue. You can change the hours and minutes as long as you keep it simple. ie. the conversion should be possible by multiplying and/or dividing by 2 or 3. With minutes add 5 to the list.

By similar logic, you want to keep the year as well. Only thing you need year for is is for communication. For internal communication anything is fine but if you make your own everyone who interacts with Earth needs to learn two systems and do conversions between them. So given that anything works for you, just use Earth years. And change years at same time as well.

You might want to change the starting date though. This only requires a simple addition and subtraction to convert. Honestly most people who need it and official documents would use dual dates anyway. That said it might be best to use standard calendar here as well. Any other starting year would probably be more important to some colonies than others and possibly cause political issues. Using Earth date by contrast unites everyone in resentment towards bloody foreigners. But you might have a unifying event that can be used.

For months... You can change them easy enough. They do not really matter that much to you. That said, precisely because they do not matter there is no reason to change them. And doing date conversions because of politics would be very unpopular and a total pain.

Additionally the calendar is basically a minor update of the one Julius Caesar set up at the very end of the Roman republic. The catholic church might have more claim on it than you do but in general you can consider it part of the common cultural heritage of all of mankind rather than an Earth thing. And it certainly would be politically much easier to sell it as such than convince everyone to adopt your own new calendar.

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    $\begingroup$ [O]ur circadian clock defaults to so close to 24 hours that it would be silly to use anything else - The human circadian rhythm is quite flexible. Submarines, for example, often operate on an 18 hour "day", and that works out just fine. $\endgroup$
    – J...
    May 6, 2019 at 12:59
  • $\begingroup$ @J... Yes, I know. That is what the "defaults" is for. ;) $\endgroup$ May 6, 2019 at 13:01
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    $\begingroup$ Yes, the point was that it is not necessarily silly to consider significant deviations from a ~24h cycle. $\endgroup$
    – J...
    May 6, 2019 at 13:06
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    $\begingroup$ Submarines, for example, often operate on an 18 hour "day" No they don't : Submarine force now 24 hr day. They finally caught up with the real world. The 18 hour day was an institutional hangover and they're hard to get rid of. The "new" 24 hrs schedule has apparently improved morale significantly according to those who monitor such things. The 18-hour thing related to watch schedules. Note the 18hr schedule did not mean the boat had it's own calendar of 487 "sub days" instead of the normal one. $\endgroup$ May 6, 2019 at 15:15
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    $\begingroup$ researchgate.net/publication/… $\endgroup$ May 6, 2019 at 20:17

Every colonized body would use 2 calendars. A local calendar and a standard calendar.

Local Calendar Days and Years have actual meaning. Days indicate when the sun rises & sets, and a year equals one season cycle. Your local calendar would be based on the body you are on; so Titan, Enceladus, Iapetus, Ganymede, Mars, etc... all have their own local calendar.

Standardized Calendar In addition to the local calendar, assuming colonists of one body communicate with colonists of another body, it would be important to have a standard calendar. And there is little doubt the Standard Calendar would be the Earth's calendar. Martians and Uranians alike would use 2 calendars, with one being Earth's calendar. It's conceivable the Saturn system could create a Standard Calendar just for themselves, but why? If they used a Saturn Calendar, there would then be 3 calendars to keep track of - and the Saturn Calendar would be as arbitrary to the people of Titan as the Earth Calendar, so they would want to keep it simple.

Local and Standard Calendars would both have importance. Day/Night of the local calendar would be very important since in low-atmosphere moons could have huge temperature shifts, and if you are on the "darkside" of the moon there would be a big difference in light. Meanwhile, business dealings, your age, when to sleep, when the next episode of Game Of Thrones comes out - would likely all be connected to the Standard Calendar.


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