On Earth, the mostly used calendar is the Gregorian calendar. This calendar has many issues though, just to list some:

  • It is not perennial. Each year starts on a different day of the week and calendars expire every year.
  • It is difficult to determine the weekday of any given day of the year or month.
  • Months are not equal in length, nor regularly distributed across the year.
  • The year's four quarters (of three full months each) are not equal (being of 90/91, 91, 92 and 92 days respectively). Business quarters that are equal would make accounting easier.
  • Its epoch (origin) is religious. The same applies to month and weekday names in many languages.
  • Year zero doesn't exist
  • Each month has no connection with the lunar phases.
  • Solstices and equinoxes do not coincide with either the beginning of the Gregorian months, or the midpoint of the months.
  • Each month has a different number of work days and weekend days making sales comparisons between months difficult.
  • and so on...

If you were to design a sensible calendar for Mars to be used by Martians from scratch, how would it look like? When would a year start? When would the year zero be? Would you still use weeks and months or something completely different?

  • 12
    $\begingroup$ Why don't you start by how you'd fix the earth calendar? What number of days fits evenly into 365.25? $\endgroup$
    – bowlturner
    Commented Jan 14, 2015 at 20:30
  • 22
    $\begingroup$ I don't understand the assumptions underlying some of your objections. Why should every year start on the same day of the week? What is wrong with a religious origin for our year numbering, let alone the names of things? Why should months be tied to lunar phases, or with solstices and equinoxes? There aren't a whole number of lunar cycles in a year, so that setup would have "issues" too. Also, the number of workdays in a month varies in part due to holidays, which have historical origins and can't be arbitrarily moved to be better distributed. $\endgroup$
    – KSmarts
    Commented Jan 14, 2015 at 21:48
  • 3
    $\begingroup$ @JohnP - It's actually a pretty elegant way of handling it. There have been cultures throughout history which have used similar mechanics, and a recurring feature of science fiction is "comp" - several minutes during the night which aren't part of a standard hour, used to sync the local day with whatever the standard is. Don't think of it as a "non existent" day - the day exists, and it's on calendars, and all that. It just exists in a separate one-day month that stands separately and doesn't count towards the 7-day cycle. It's weird only because we're not used to it. $\endgroup$
    – Bobson
    Commented Jan 14, 2015 at 22:19
  • 4
    $\begingroup$ Read KS Robinson's Red Mars -- has a great Martian calendar outlined at length $\endgroup$ Commented Jan 14, 2015 at 23:57
  • 7
    $\begingroup$ I think it's important to look at why the Gregorian calendar was introduced. Earlier calendars did focus on things like moon phases, or seasons, or being "regular"... unfortunately, all of them "drifted", in the long run. You cannot cover all of the moon-months, the sun-year, the solstices, etc. etc. in one single calendar. (Besides, I think it's funny that the epoch is Christian, the weekday names are mostly Norse / Germanic, the month names Roman, and the biggest western holiday actually a bastardized solstice. ;-) ) $\endgroup$
    – DevSolar
    Commented Jan 15, 2015 at 9:59

10 Answers 10


Assuming there are (or were) native Martians, what makes you think their calendar would be any more rational than ours? They too, would have whatever religious/historical/accidental quirks that their cultural development left, which would complicate it just as much as any human calendar.

That said, if you're asking about designing a rational calendar from scratch, you can't. Celestial mechanics don't line up in any periodic ways, so you're going to have to just choose some of your goals and abandon others.

For example, the "year" on Mars lasts 668.5991 "sols" (where a sol is the Martian equivalent to a day). No possible calendar that's divided into whole units of sols can possibly account for that, which means you're left with a floating 6/10 of a day. In other words, in three out of every five years, you'll need to insert a sol, or your calendar will drift. Drifting is not automatically a bad thing! The Islamic calendar is entirely lunar based, doesn't account for drift at all, and months (with the associated holidays) slowly cycle around the year. But this does preclude tying the year to any form of season.

Separately, a Martian calendar probably wouldn't have any concept of months. Our concept is originally based on the moon's cycle (although under the Gregorian calendar it doesn't match up at all), but the moons on Mars move way too fast to have inspired that connection. Instead, they'd probably be used to track portions of the day ("I'll meet you in three passes of Phobos...") Likewise, the seven-day week is a totally religious concept which has no astronomical analogy, so while there might be some equivalent, it'd be totally culture-based. (The traditional Chinese calendar didn't have a week grouping, and the French Revolutionary Calendar had 10 day weeks.)

So feel free to make up any odd rules and/or adjustments you want - the universe isn't going to provide a "sensible calendar" ready for use.

  • $\begingroup$ The seven day week is used in some religions. But is there any particular reason to think that is the origin of the seven day week? The seven day week could be older than those religions. It also could have its origins in astronomy since it happens to be approximately one quarter of the time it takes the Moon to orbit the Earth. $\endgroup$
    – kasperd
    Commented Jan 14, 2015 at 22:18
  • 3
    $\begingroup$ @kasperd - Like much of our timekeeping (including dividing the year into 12 months, the day into 12*2 hours, and hours into 12*5 minutes), it actually derives from the Babylonian/Sumerian system. In this case, they had a holy day every 7 days after the new moon started a new month, with the "extra" days just getting lost when a new month started and the count started over. The concept of "7 days regardless of month divisions" is a later innovation. $\endgroup$
    – Bobson
    Commented Jan 14, 2015 at 22:29
  • 1
    $\begingroup$ About Moon and months. Martian moons are too small to induce tides. Earth Moon has impact on Earth which is significant enough to notice (apart from lightning some of the nights), and thus as another incentive to tie it into calendar, Martian moons, not so much. (implicit assumption - we are talking about theoretical liveable Mars with atmosphere and liquid water) $\endgroup$
    – M i ech
    Commented Nov 30, 2016 at 0:26

The Earth calendar was inspired by the need to track seasons and the harvest; one speculation I read once is that a Martian calendar would be inspired by Earth-Mars transfer windows, because supply ships from Earth would arrive in a pretty regular pattern of about one group every 760 (Martian) days.

So perhaps the Martian calendar would ignore astronomy entirely and be based on nineteen months of 40 days each.

  • 1
    $\begingroup$ Love the idea. Very practical until the day we discover and use faster travel. Would be nice if you could find the source for that. $\endgroup$
    – phresnel
    Commented Jan 15, 2015 at 9:55
  • 1
    $\begingroup$ @phresnel - Very true. But even after it's no longer practical, it'd still probably be used. As witness our current calendar and the point of this question :) $\endgroup$
    – Bobson
    Commented Jan 15, 2015 at 15:48
  • $\begingroup$ I was going to suggest this as well. As a note though, the Earth-Mars Synodic period is ~780 Earth Days, and ~760 Mars Days. $\endgroup$ Commented Nov 26, 2018 at 22:13

You could start by reading Timekeeping on Mars and Darian calendar on Wikipedia.

The Martian year has a length of 668.5991 sols (martian days). So you start with the same leap years issue that is found in Earth's Gregorian calendar. Also, Mars moons (Phobos and Deimos) are unusable to determine months or even weeks, as they orbit the planet faster than once a sol. So seasons are by nature uneven on Mars. The start of the year should most probably be Ls=0

  • $\begingroup$ What does "Ls=0" mean? $\endgroup$
    – svick
    Commented Jan 15, 2015 at 16:52
  • $\begingroup$ Ls (Solar Longitude) is the angle that Mars has walked in its orbit around the Sun. Ls=0 is the Northern Hemisphere Spring Equinox. See www-mars.lmd.jussieu.fr/mars/time/solar_longitude.html Ls=90, Ls=180 and Ls=270 are the other three season change points. $\endgroup$
    – Envite
    Commented Jan 15, 2015 at 20:38
  • $\begingroup$ I came here to tell about the Darian calendar, too. $\endgroup$ Commented Feb 15, 2023 at 9:26

Timekeeping based on Mission Elapsed Time

Most space missions do their timekeeping based on mission elapsed time, which starts with the liftoff of the rocket.

A mars colonization project would be such a space mission. So it would be sensible to assume that the MET timekeeping is continued from launch to transfer to landing to colony building and so on, so the Mars timekeeping might never stop to use the launch of the first rocket of the first mars colony as the reference point.

Timekeeping based on Mars landing

However, the Mars rover missions Sojourner, Spirit/Opportunity and Curiosity planned their missions on Mars according to Mars days ("Sol"s) elapsed since their landings. Using Mars days as the basic unit of the calendar would be more useful than sticking to earth-days, as it matches the day- and night cycle. The landing of the first Mars colonists would also make a good reference point for the beginning of the calendar, as it marks the beginning of human civilization on Mars.

  • $\begingroup$ The most famous form this takes for mission elapse is the famous countdown notation T-10 seconds (10 seconds from mission start or -10 seconds into the mission) from count down. Mission Start is always 0 with no T and time beyond the Mission start is noted as T+10 seconds. Typically, NASA used hours. $\endgroup$
    – hszmv
    Commented Nov 2, 2018 at 20:41

I would have 6 day weeks, 36 day months, and 216 days years. Why should my calendar be tied to the orbit of the planet? I would base everything off of base 6 since it's divisible by 2 and 3, which are the most common ways that something needs to be divided. The planet would orbit approximately once every three years, but would be off by slightly more than 20 days.

Living on mars, I wouldn't be planning life around the annual cycle of seasons, since I live indoors and the planet is far too cold to have any naturally growing plants. My colonists would mostly live based on hydroponics in lighted areas, so I could even vary the 'daylight' in accordance with some artificial 'seasons' if that was healthy for them.

If my colonists wanted to track approximately where in the martian seasonal cycle they were, each year could be described with where it fell in the season. 'Year 53 is a spring/summer year', for example, would describe that the length of the days would grow longer throughout the year, coming to a maximum in the summer.

  • $\begingroup$ Not a bad idea :) $\endgroup$
    – Envite
    Commented Jan 14, 2015 at 21:48
  • 1
    $\begingroup$ If we were on Mars I'd reject that idea ;) After all there are seasons. They would even become more pronounced after terraforming efforts (however long that would take of course...) I therefore would strongly suggest a calendar fixed to orbital mechanics, meaning a martian year is the time it takes to go once round the sun. Consider days (although being pretty much the same length as on earth), would you suggest not tieing it to a full rotational cycle of the planet? $\endgroup$
    – Ghanima
    Commented Jan 15, 2015 at 7:54
  • 1
    $\begingroup$ An interesting aside related to this idea: There's a theory that the biblical patriarchs' long lives are because the definition of a "year" changed. For the first generations (ages in the 700-900's), it was actually a month (age/12). For the later ones (ages in the upper 100's), a "year" was 4-6 months (a season). I don't know how well it holds up, though. $\endgroup$
    – Bobson
    Commented Jan 15, 2015 at 15:59

So this is something I came up with in chat a while back and mostly just want to show off:

enter image description here

The months are named for the children of Mars. The sols (Martian days) of the week are names of Mars in different languages.

A week is 6 sols long. Because of the 30 sol months and six sol weeks, the first of the month is always Simudsol and the 30th is always Nirgalsol, which is nice because when someone says that you're appointment is on the 15th, you know automatically what sol of the week that is without having to ask.

The one oddity is the month of Ares, which is 8 sols long, doesn't use names for the sols, and is the equivelant of Christmas and Thanksgiving combined with New Years, with a bit of "yay we aren't on Earth any more" 4th of July thrown in. Basically an 8 sol party.

I'd try to time landfall so that it's on a solstice or equinox, just to start the year off right.

  • $\begingroup$ Four divisions of forty-one days, twelve divisions of forty-two days will fit very nicely. $\endgroup$ Commented Nov 2, 2018 at 19:25
  • $\begingroup$ @ErkinAlpGüney If you want to figure out some more details and type that up as an answer that's fine. I like my idea though. It's why I need to colonize Mars first, just so I can start the calendar off right. $\endgroup$
    – AndyD273
    Commented Nov 2, 2018 at 19:32
  • 1
    $\begingroup$ Nice job actually building a calendar that directly answers the OP's request, instead of challenging their assumptions! $\endgroup$
    – Bobson
    Commented Oct 26, 2021 at 13:48
  • 1
    $\begingroup$ @Bobson Since it doesn't look like I'm going to be the one to colonize Mars first, I wonder if I can get Elon to take a look at it. I had kind of forgotten about this, so thanks for bringing it back to my memory $\endgroup$
    – AndyD273
    Commented Oct 26, 2021 at 14:13
  • $\begingroup$ @AndyD273 I'd forgotten my own answer until someone upvoted it. It's always nice to go back and revisit old answers. :) $\endgroup$
    – Bobson
    Commented Oct 27, 2021 at 15:10

As a kid, (before even calculators), I drew up calendars for every planet, (back when we still had 9). I would initially base months on the orbit of the largest plausible satellite. I recall when I got to Mars and found both moons orbited the planet in about a day, so there would be no "months". I had a fallback where the local culture would develop a calendar divided up by the position of constellations, (like the zodiac for those in planes similar to the Earth), maybe even based which ones appeared relative to sunrise and sunset. I think I eventually came to the conclusion that a zodiac-based calendar could be used on every planet to indicate its angle relative to the sun. Division of constellations could be arbitrary, but 12 is suitable for integer division.


I have a suggestion of the timekeeping on Mars:

0. TL;DR

Divide the year into 23 months of 29 sols each (with a few exceptions). One benefit is that 12 months on Mars will be almost the same as 12 months on Earth.

1. Background:

One day on Earth is 24 hours = 86,400 seconds.

One year on Earth is 12 month. The months length differs between 28 and 31 days.

One Julian year is 365.25 days.

One day on mars is 24 hours, 39 minutes and 35 seconds = 88,775 seconds. (Give it or take some parts of a second).

One Martian year is around 687 Earth days = 668.5991 Martian days.

2. Proposal for a Martian chronology

2.1 Clock
We keep the earth seconds as a base measurement for time. This has a lot of advantages, especially then many SI units and most technique that handling time in some way use seconds.

2.1.1 The day
The Martian day is 88.775 seconds, witch is divisible with 25. Therefore it is wise to dividing it in 25 Martian hours that is 3,551 seconds long.

The Martian hours can be divided by 67 × 53 (67 minutes by 53 seconds, or 53 minutes by 67 seconds) Leap seconds are frequently needed in Mars with a timekeeping in Mars, hence 67 minutes of 53 seconds are a better choice

2.2 Calendar

2.2.1 The year
The Martians will be dependent on Earth for a very long time. Therefore it is wise to keep units to be close to the one on Earth.

The length of the year is very close to 668.6 Martian days, which means that 60% of all years needs to be leap year. My suggestion is that years 0, 2, 4, 5, 7 and 9 of every decade have to have 669 days, and the rest of years to have 668 days.

2.2.2 The months The length of months
The Martian year should be divided in 23 Martian months with most Martian month to be 29 Martian days long. Let's take a look of the months sorted by lengths by seconds:

  28 Earth days  : 2,419,200 seconds
  29 Earth days  : 2,505,600 seconds
  29 Martian days: 25,74,475 seconds
  30 Earth days  : 2,592,000 seconds
  30 Martian days: 2,663,250 seconds
  31 Earth days: : 2,678,400 seconds

Almost all months should be 29 Martian days long. Only month 11 should be 30 days long. The last month on the year has the leap day.

  Month 1: 29 Martian days
  Month 2: 29 Martian days
  Month 3: 29 Martian days
  Month 4: 29 Martian days
  Month 5: 29 Martian days
  Month 6: 29 Martian days
  Month 7: 29 Martian days
  Month 8: 29 Martian days
  Month 9: 29 Martian days
  Month 10: 29 Martian days
  Month 11: 29 or 30 Martian days
  Month 12: 29 Martian days
  Month 13: 29 Martian days
  Month 14: 29 Martian days
  Month 15: 29 Martian days
  Month 16: 29 Martian days
  Month 17: 29 Martian days
  Month 18: 29 Martian days
  Month 19: 29 Martian days
  Month 20: 29 Martian days
  Month 21: 29 Martian days
  Month 22: 29 Martian days
  Month 23: 30 Martian days ** 30 Days on the last month on the year.

This calendar has several benefits. The best argument is that the length of 12 Martian months is quite close to 12 months on Earth, which will make it easier to do rough calculations and cross referring calendars:

1 earth year, no leap year : 31,536,000 seconds 1 Julian year (365,25 days): 31,557,600 seconds 1 earth year, leap year : 31,622,400 seconds 12 Martian months, long : 30,982,475 seconds 12 Martian months, short : 30,893,700 seconds

The difference between 12 Martian months and 1 Julian year is ~2% (around 1 Earth week).

The Earth Calendar (Western calendar) has a lot of bigger differences built in. The earth months is usually a bit longer, but not always:

February to April is 89 Earth Days = 7,689,600 seconds. Three Martian Months i usually 7,723,425 seconds. The months names
The original Julian calender on Earth had Mars as it's first month. Therefore has the months "September", "October" "November" and "December" the meaning in Latin 7, 8, 9 and 10. Unfortunately this changed. so the start of the year became January. That made the months quite a bit unlogically.

With starting a new Martian calendar, we have a unique chance to repair that mistake. Therefore I suggest the Martian year to start with Mars, which also is the name of the planet!

After the Martian month the rest of the Earth months names follow, and then one can use different celestial bodies or the Latin name of the numbers:

                                  Earth name Latin number  Celestial  Proposal
  Month 1: 29 Martian days        Mars       Unuber        Mars       Mars
  Month 2: 29 Martian days        April      Duober                   April
  Month 3: 29 Martian days        May        Triaber                  May
  Month 4: 29 Martian days        June       Kvartober                June
  Month 5: 29 Martian days        July       Kvinkober                July
  Month 6: 29 Martian days        August     Sexber                   August
  Month 7: 29 Martian days        September  September               September
  Month 8: 29 Martian days        October    October                  October
  Month 9: 29 Martian days        November   November                 November
  Month 10: 29 Martian days   December   December                 December
  Month 11: 29 Martian days   January                             January
  Month 12: 29 or 30 Martian days February                            February
  Month 13: 29 Martian days                  Triadecimber   Jupiter   Jupiter
  Month 14: 29 Martian days                  Qvartdecimber  Saturn    Saturn
  Month 15: 29 Martian days                  Qvincdecimber  Uranus    Uranus
  Month 16: 29 Martian days                  Sexdecimber    Neptune  Neptune
  Month 17: 29 Martian days                  Septemdecimber Pluto     Pluto
  Month 18: 29 Martian days                  Octodecimber   Sol       Sol
  Month 19: 29 Martian days                  Novemdecimber  Mercury  Mercury
  Month 20: 29 Martian days                  Vigintiber     Venus     Venus
  Month 21: 29 Martian days                  Unvigintiber   Terra     Terra
  Month 22: 29 Martian days                  Duovigintiber  Phobos    Phobos
  Month 23: 30 Martian days                  Tresvigintiber Deimos    Deimos

2.3 The weeks The weeks can be the same as on Earth:


Possibly, if you want to make the calendar simple, you can have double or triple Sundays in the end of the month to make every month begin with a Monday.

2.4 Then should the new year be?
Most ancient earth calendars have there new years in the spring equinox and that is also a good idea for the Mars calendar. The first day after the equinox should be the first day on the year. The hemisphere the first settled landing is on decides what hemispheres spring equinox should be calculated from.

2.5 Which year should be year 0?
It would be properly to start counting the years after the first permanent (or at least planned to be permanent) settler set his foot on the surface of Mars. That should be the year 0. Unlike the Earth calendar—the Western calendar does not have any year 0 in the calendar— the Martian should have a year 0 in its calendar. It makes it more easy to calculate years before the setting of mars. It will also have the consequence that some day in the year 50, it will be 50 years after the settling, and not 49 years.

2.5. How about the clock? When should be midnight be?
The midnight 00:00:00 should be like on the Earth, when the Sun has its lowest position on the night. Because of Mars will have 25 hours per day, the Sun will have its highest position at 12:33:27.

  • 1
    $\begingroup$ 60 can be easily divided by 2, 3, 4, 5, 6, 10. 53 and 67 does not have this basic feature. Your clock makes things so common as "half an hour" hard. That's bad. That's why this wouldn't catch on. $\endgroup$
    – Mołot
    Commented Mar 17, 2017 at 14:32
  • $\begingroup$ Also, please note that Earth months are 2 or 3 syllables each, and yours, well, longer. Also, abbreviations are relatively easy and unambiguous. Not much room for mistakes. In your calendar, you have two "Sex.", and similarities like Kva. and Kvi. next to each other. Really bad idea. $\endgroup$
    – Mołot
    Commented Mar 17, 2017 at 15:41
  • $\begingroup$ @Molot In your calendar, you have two "Sex.", and similarities like Kva. and Kvi. next to each other. Really bad idea. English-based Earth calendars have months abbreviated Mar and May as well as Jun and Jul. Meanwhile, you have the months of Sept, Oct, Nov, and Dec being the ninth, tenth, eleventh and twelfth months instead of being, as their name suggests, the seventh, eighth, nine, and tenth months. (They were the seventh, eighth, ninth, and tenth months before Pope Gregory XIII's reforms.) $\endgroup$
    – Yoshi Bro
    Commented Mar 22, 2017 at 8:51

Since we can presume that Mars will be part of a larger solar economy and political system (settled by humans), then the calendar needs to be aligned to that of everyone else in the Solar System. This also makes things like banking, internet connections and space navigation easier, since everyone is running off the same conventions.

There is an already existing system, called POSIX time, based on the idea of counting seconds from the instantiation of UNIX. Second 0 is Jan 01 1970, which by odd historical coincidence is very close to Neil Armstrong Day (20 July, 1969). IF everyone is on POSTIX time, then historical events, timestamps and everything else can be recorded with great accuracy and will be the same for everyone using that calendar.

We all know where we were on 1315730760


They could combine cycles, just like on earth

On earth, 19 years measured by the sun almost exactly equals 235 months measured by the moon. This is known as the Metonic cycle and was the basis for both the hebrew and babylonian calendar, as well as a "runic calendar" invented in Scandinavia in the middle ages. An attractive property of the calendar is that it can be used to predict eclipses. It's also the basis for predicting the date of Easter.

We could imagine that the martians devise a similar calendar, based on the sun and its two moons. We know the following:

Mars orbital period: 686.971 * 24 hours
Phobos orbital period: 7 hours, 39.2 minutes
Deimos orbital period: 30.312 hours

It turns out that 5 martian orbits equals exactly 10 771 orbits of phobos while two martian orbits equals exactly 1088 orbits of Deimos. Therefore:

The full system of 3 orbits completes one cycle in 10 martian years. 

Usage: Really great for predicting the position of Phobos and Deimos with respect to the stars and the sun. Several eclipses could be predicted.

But note how this does not take into account the rotation of mars. You'll know the position of the sun with respect to the moons, but not if the sun is above the horizon. We don't know if the martians cared much about cycles of light and darkness, but here too we can make metonic cycles.

The martian day is 24 hours 37 minutes and 22 seconds long 

We then have:

474 martian days equals exactly 1525 orbits of Phobos
309 martian days equals exactly 251 orbits of Deimos

It would be fun to have two competing martian calendars based on Phobos and Deimos respectively, but you can also combine them, to unite the warring tribes:

The cycle of martian days and moon orbits completes in 16274 martian days

Usage: The martians could use this calendar to predict repeating patterns of how the moons rises and sets in the sky.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .