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There are many different calendar systems on earth. Of course, since a fictional world very well may differ greatly from Earth, it makes sense for them to have their own calendar.

What are various things that should be kept in mind or thought about when designing a calendar system for your world?

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Cosmology and astronomy

Calendars will have a strong dependence on the cosmological perspective of the people creating it and their astronomical capabilities, since celestial mechanics determine the day-night cycle, seasons and year length. Obviously they will count the seasons and days without having to look at the sky for a number, but most any culture will have rudimentary sky observation information, the least of which will be constellation movement tracking.

While astronomy on one hand is obvious as a factor, I mention their cosmological perspective because it can strongly affect how they number things and how complicated and arbitrary their system is. Look at ancient Greek epicycles and Phlogiston for a couple of ideas of how reality gets messed up because you just have to have your planet in the middle of the universe and the universe perfect. If a culture has an idea of the world that, for example, disallows the year to be anything close to 400 days, even though the actual year is 423 days, they might come up with a year of 846 days and explain that "everything is paired" and "that's why we have 8 seasons and the mid-year festival".

Mythology

The effect of their cosmology is modified and further compounded by mythological beliefs - not only the naming but also the scales involved will be based on what their perspective of the universe is. A culture that believes they were created by deities a few millennia ago, will likely use that arbitrary point in time as the start of their calendar. If they have chosen an arbitrary date for the same event, they might choose to start the year from that date or place it in the middle of the year. These can be completely arbitrary.

Mathematics and counting

Their mathematical capabilities will also affect the form of the calendar. A culture more adept at geometry may prefer to create a calendar that allows geometric relationships when laid down, to determine the relationships of their astrological ideas and the seasons. A culture that has developed complex algebraic concepts might prefer a calendar will composite numerals that compress more information into the structure. Most any calendar will have a simple counterpart though, to make farming use easier.

Examples

Some examples of calendar variation are these:

  • In the Gregorian calendar year enumeration begins at the religion-based year that the Christian Messiah was born, yet it is used today by other cultures and people who don't share the religion itself. The Hebrew calendar, despite strong cultural relationships with Christianity, beings its enumeration much earlier, on the order of millennia. The Mayan calendar uses a mythological starting point, much like the others, that is also millennia before the Gregorian year zero.

  • The Gregorian calendar still uses the Roman names for months, many of which are references to the Greek pantheon correlated to the planets and others being the names of Roman emperors (or coinciding with them). While this is a minor point, the naming can depend on a long history of cultural takeovers and shifts, as well as the degree of reverence that the people place of different parts of their world. The Chinese use animals for the 12-year astrological cycle for instance.

  • Some calendars have numerous epochs. The Gregorian calendar can be considered to have two: BC and AD, but the Hebrew calendar has multiple ones. The fictional calendar in the Elder Scrolls universe also had multiple epochs, that begin and end based on the predictions of the scrolls and historical events. This seems to be mythology dependent, much like determining a starting point.

  • The number system can differ - we use decimal, or base-10, but the Mayans used base-20. Among ancient cultures, there were many kinds of numeric bases for their counting and there was little consistency. The Sumerians used base-60 and I've read of base-12, base-24 and others being used, sometimes by the same cultures. A good example of mixing is the Imperial System for measurements: A foot is 12 inches, a yard is 3 feet, a mile is 1760 yards. While cultures using the system didn't mix bases, if there was a complex calendar system, it may well use different bases for different layers, to compress representations.

Oddities

Some peculiarities:

  • The Mayans had 2 kinds of years: lunar and solar years - the lunar year lasted 260 days while the solar roughly 365 days. These formed a cycle that completed every 52 solar years. So, perhaps by their perspective, there were long and short years. They also used a different count for long periods of time and short periods of time. Many cultures have so called "lunisolar" years.

  • Many calendars, such as the Chinese and the Gregorian, have fudge factors to make sure alignment is maintained over inaccuracies that collect over long periods of time. For us it's February 29 (nearly) every 4 years, for the Chinese it was a month or two added every now and then.


Summarizing, the information you should know before making a calendar is:

  • Actual astrophysical orbit, including other planets and the moon(s), if any exist
  • Creation and cosmological mythology of the culture
  • Their mathematical system and capabilities
  • Their cultural activities that use the calendar
  • Their history, which will determine calendar characteristics that stay stable over time because, once established, they have no reason to change (such as year zero)

Wikipedia has a lot of information on the aforementioned calendars and is a good place to start for more.

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  • $\begingroup$ Good answer. A few comments: What epochs does the Hebrew calendar have? Also, the Hebrew calendar's start date is earlier than the Christian one, not later (you said it starts "sooner", which to me means "more recent" -- maybe not what you meant?). And by the way, saying that the Hebrew calendar has a cultural relationship to Christianity is backwards; I would suggest just removing that tangential comment. Thanks. $\endgroup$ – Monica Cellio Oct 14 '14 at 12:39
  • $\begingroup$ @MonicaCellio I've added the wikipedia links I neglected when I posted this and clarified the "sooner" part. However, I disagree on the "cultural relationship" thing - it isn't backwards because that statement has no directionality in the first place - it's not tangential because I'm using it as a contrast to highlight the differences that can arise despite cultural relationships. $\endgroup$ – mechalynx Oct 14 '14 at 13:07
  • $\begingroup$ Thanks for the edits; that's much better. (What about the epochs? I don't know what you mean by that with respect to the Hebrew calendar. Granted that this is all minor.) $\endgroup$ – Monica Cellio Oct 14 '14 at 15:26
  • $\begingroup$ @MonicaCellio On the calendar thing, what I mean is that I'm establishing that the two cultures are related, much like how two people may share a culture - there isn't a "backwards" because it's a mutual thing. The epochs are much better explained by the wikipedia article or others, I have no idea how they work, other than knowing they exist, which is why I didn't elaborate on them. $\endgroup$ – mechalynx Oct 14 '14 at 15:33
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My system for generating a calendar is fairly simple:

  1. When does it start?
  2. What are you counting?

Let's start with the first one, when your calendar starts is often defined by your civilisation's history. It may be the foundation of the nation, the date that the aliens were defeated, the date the god(dess) created the world. This will be very specific to your world.

Second, what are you counting? The traditional is a year, one rotation of the world around the sun but there's no reason you can't count seasons, days or any other unit. The unit selected should reflect the society using it, an ancient civilisation are unlikely to use precisely measured atomic periods, they're more likely to use natural and observable times such as a year. Equally a multi-planet empire is unlikely to use the orbit of one of it's planets as a measure... what about the others!?

As for more granular measuring days is a very obvious one (sunrise/sunset is very hard to ignore regardless of your technological level) but why not count hours since sunrise?

  • The number of shortest days since the last great volcano eruption
  • The number of seasons since the foundation of the nation
  • The number of winters since the "beginning of the world"

Finally note that a lot of calendars have some kind of scaling letters. AD/BC, The 1st, 2nd, 3rd or 4th ages. To a little bit of spice why not add in some letters to your date?

142nd day of 194 DL (Dawn of Light or some equally vague/poetic units).

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  • $\begingroup$ but if you don't have seasons (no axial tilt and no eccentricity of the orbit) then counting years doesn't make sense until astronomy comes into play $\endgroup$ – ratchet freak Oct 13 '14 at 11:47
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    $\begingroup$ @ratchetfreak I was actually going to say that astronomy was for newer civilisations but stopped myself. To the best of my knowledge many older civilisations mayans/egyptians/greeks had very detailed knowledge (or at least measures) of astronomy. That's why I believe counting years in ancient civilisations is ok. $\endgroup$ – Liath Oct 13 '14 at 11:49
  • $\begingroup$ One place where there are no seasons is on a sublight starship. Will they keep counting out the irregularities of one of Earth's calendars? Or just count seconds, kilo- mega- abd giga-seconds? Or go over to 12 months of 30 days each? Or even adopt longer days (100 k secs?) which might be physiologically easier without bright sunlight to forcibly synchronizer our metabolisms? $\endgroup$ – nigel222 Feb 8 '16 at 15:03
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The main reason to have a calendar is to predict important points in time (when to sow, for example). Those points are often tied to the year. Identifying points in the year is most reliably done with astronomical markers (for example, IIRC the Egyptians used the appearance of Sirius above the horizon as sign to start sowing).

Also very obvious observable periodic changes (the moon phases!) will likely enter into the calendar (if living on the sea, the moon phases will also intimately be linked to the tides; note however that adoption of lunar cycles may also have been driven by the fact that the female fertility cycle is very close to it).

The base unit of a calendar normally is the day (when there is no night-day cycle, I can imagine another base unit being used). Anything below that base unit is time keeping, and not related to the calendar (except for the defintion when a day begins; the usual calendar uses midnight — the middle between sunset and sunrise — while the Jewish calendar uses the sunset as start of the day.

After the base units are set, numeric properties play an important role. For example, the moon cycle is between 28 and 29 days. 28 = 4*7, while 29 is prime. Usually divisible units are preferred; the 7-day week most probably is related to the 28-day moon cycle. On the other hand, there are about 13 28-day months in the year, which again is a prime; however, make it 12 months of 30 days, and you have two quite divisible numbers; and even better, both are obtained by dividing the same number, 60, by a small number (2 or 5).

Of course, that way you inevitably get a mismatch of your cycles (for example, a year is between 365 and 366 days long, with 13 28-day months you get 1 to 2 days too little, with 12 30-day months, you even get 5 to 6 days too little. That can be solved by having a few "special days" to fill up the year (if the priests set the number of special days according to astronomical observations, this also saves you the trouble to make complicated rules like leap years), or it can be solved by making some sub-units a bit longer or shorter (like our calendar, where we have some months with 30 and some with 31 days, with a mostly regular pattern — the irregularities (and the much shorter February) are due to later modifications of the calendar by Roman emperors (for example the emperor Augustus — after renaming the month Sextilis to August — wanted to have his month (which up to then had 30 days) to also have 31 days, so he gave it another day, exchanged the month lengths of the following months (maybe because otherwise there had been three 31-day months in a row), and took the extra day away from the February (which already had a differing length anyway).

The latter also shows a third influence on the calendar: Messing with the calendar for various reasons, from actual time-keeping reasons (adapting the leap year rule to better fit the astronomical year, after the difference becomes problematic, as done with the Gregorian calendar) over cultural/religious reasons (resetting the date number from Roman to Christian calendar, based on the claimed birthday of Christ — the fact that Christian year numbers are divisible by four exactly if Roman year numbers are, which allows for the same leap year rule to result in the same leap years, makes me suspect that the birthday of Christ wasn't the only consideration when fixing the starting point) up to just self-esteem of emperors (the Augustus example above).

So in short, the following things affect the calendar:

  • The cycles important to daily life, as well as easily observable cycles in nature (especially astronomical ones).
  • Numeric properties, especially divisibility (but also the religious or superstitious significance of numbers may enter here; OTOH that often is also derived from the divisibility).
  • Intentional changes to an existing calendar, often with a goal not directly related to timekeeping considerations.
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Let's start with the most popular calendar in the English speaking world, which has 12 months broken into days; the number of which range from 28 through 31. Those days are then organized into periods of 7 day weeks.

One would conclude that by dividing seven into the largest number of days in a month one would determine that the maximum number of horizontal lines of weeks would be four and a bit, or five weeks (rounding up).

31 / 7 = 4.43, rounded up to 5

The truth is that since the calendar makeup is comprised of some prime numbers, the months will often start on different days of the weeks and end on them on different days of the week most often too. Therefore, six lines of dates are needed to lay out a month without resorting to squishing more then one day in a single square.

Now, we live in a simple solar system and have a single moon revolving around a single planet with a single star at the center. Imagine how different a calendar might be if it took in the influence of a double moon (such as Mars has). Or if our Earth and some other planet revolved around each other on our annual orbit... or if we had a binary star at the center of our solar system. One other thing that may affect a calendar's layout would be if our planet revolved around the sun with its axis at 90° to the its travel direction around the sun.

Our calendar is affected by our culture as well and needs to address the strict observance of religious holy days, and political observances. It's interesting to notice that in spite of the majority of this planet is covered in water, it is left to imposing a tide chart onto the calendar rather then allowing the time chart to drive the organization of the calendar. Additionally, any agricultural influence is made to superimpose itself on top of the religious/politico calendar too.

Many of Earth's cultures did fine and existed for centuries without a calendar at all; the American Indians for example. It bears reminding ourselves that some cultures in South America exist today with only a very vague sense of time and no counting system beyond the numbers two or three. So if I said to you, "I will be gone a long time." It is up to you to decide when the "long time" has passed; be it days, weeks of days, or even months or years. A very tricky thing when one barely imagine past tomorrow or the next day at best.

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  • $\begingroup$ I'd say any religious/political influence is on top of the agricultural one, not the other way round. $\endgroup$ – celtschk Oct 14 '14 at 7:29
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In general calendars are shaped by events that can be seen or observed and for the most part these are astronomical events.

In the tropics a year has little meaning but in temperate and polar regions it is a really big deal, so it can be expected that that will take an important role.

Days are the other obvious division - light and dark marking periods of activity.

Between the year and the day though there is a huge range of time, so it makes sense to split that up. Obvious approaches are seasons, or if you have a moon then phases of the moon and tide again is very visible and dramatic and obvious.

A planet with multiple moons or multiple suns would be much more complex but really all you need to do is stop and look at your planet and its environment. What regular events would its inhabitants see and how would they use those events to measure the passage of time.

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One important things wasn't mentioned: the calendar can be heavily influenced by politics too. Each "era" starts with the year new sovereign or dynasty started their rule. Ancient Egypt, or even Old Testament Izrael used this system. A randomly picked Old Testament example: "In the eighteenth year of the reign of Jeroboam son of Nebat, Abijah became king of Judah..." (1Kings 15:1)

This system has obvious drawbacks. It requires maintaining long lists of kings and lengths of their reign to make counting years over a long span possible. This is not exact, especially if some rulers are "politically incorrect" enough to be erased from history (Achnaton and the following two pharaohs make a good example). And this requires the culture to be dominated by one or at most two (Izrael + Judah) sovereign lineages, otherwise it is a mess even for contemporaries.

On the other hand, with low literacy level where only priests and kings' scribes need to count years exactly, this system was quite practical for the administrative (bureacrats knew who were the last few kings and how long did they reign), and it pleased the kings, in some cultures seen as human gods. The priest may or may not have an alternative calendar better suited for long-range year counting.

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