Struggling to come up with a good calendar system for your world? Maybe you were originally wanting to base it off our established Gregorian Calendar, but are now not sure if that’s a good idea for your world. Or, does your world have two moons, which is now complicating the situation – so what’s in a year? How do you come up with your world’s calendar?

This informative guide for worldbuilders and writers will go into different scenarios when it comes to calendar creation and how to approach the process for your world.

Topics included in this guide:

• Is a Calendar Necessary
• The Current Gregorian Calendar
• Solar vs. Lunar Calendars
• Calendars in Space – Interplanetary Calendar Creation
• Calendars on a Multi-Moon World
• Calendars on a Multi-Sun World
• Naming your Months
• Important Calendar Creating Notes

Is a Calendar Even Necessary for your World/Story?

Let’s address the question of whether or not you even need a calendar first. You might have really gotten deep with your world and the stories within, and have already begun thinking about how your world’s people track time. However, if your world is meant for an audience, you need to be careful about how you approach calendar creation.

When deciding on calendar creation for your story, start without it. You want your reader to enjoy the story and the world you’ve built without distraction. So try to get away with not having a calendar. This is easy to do when you reference seasons, “Joe woke to a bite in the air and the change in the sun. His heart surged, anticipating the coming harvest and fall festivals.”

Maybe, you need to reference the passage of a month, you can still call it a month. If you want your reference to be unique to your world, call it a moon (which isn’t very unique).

Very rarely will you actually need to reference a finite calendar system, but if your story actually calls for it, then devise an internal logic system for yourself, and only present relevant aspects of it (month names, calendar type) when it becomes necessary.

Sci-fi stories with interplanetary references are going to require the use of a calendar system more than a fantasy, one-world story. Common points of references such as seasons, the sun and moon are out the door. What complicates this more is when more than one planet is involved, or a story takes places on a ship.

If you have decided that calendar creation is still necessary for your world and the stories you’re working on, then the rest of the guide will provide you with more information to help you create the best system.

Understanding the Gregorian Calendar

In October, 1582 Pope Gregory XIII introduced a calendar that would become the most widely used and internationally used calendar today, the Gregorian Calendar. It was not the sole invention of the pope. Instead, it was an improved version of the Julian Calendar, named for Julius Caesar. The changes made improved upon the Julian Calendar, making every fourth year a leap year; however, this excluded years divisible by 100 unless the year is further divisible by 400 (2000 is a centurial leap year, but 1900 would not be).

But none of that really means a whole lot when it comes to devising your own calendar. What is important to understand about the Gregorian Calendar is that it enabled holidays to be pointed to a specific date in the year. Further, it prevented seasonal drift where holidays rooted in lunar calendars will drift over time into other seasons. This is because the Gregorian Calendar is a solar calendar, and will track with the earth’s revolution around the sun.

Solar vs. Lunar Calendars

Of course, the names are going to explain a little bit. A solar calendar is going to pertain to how the earth rotates around the sun. And a lunar calendar pertains to the moon’s orbit around the earth.

The problem is that there is not an even ratio between the moon’s orbit around the earth and the earth’s orbit around the sun. Overtime, there is going to be seasonal drifts of holidays and dates. A fictitious month based on a lunar calendar might fall in the winter season one year and a few years later fall in the summer season.

Why would anyone use the lunar calendar when the solar calendar makes more sense? To answer that question, you need to consider where on earth people groups live and how important seasons are going to be. Nearer to the equator, seasons are going to impact a region far less, and tracking moon phases is pretty easy to do. Regions located farther away from the equator will experience very defined seasons, and therefore, the solar calendar is going to be most useful.

When it comes to your world, think about how far races are from your world’s equator. It might even be more natural to have two calendars employed in your world: a solar calendar for the people of the north, and a lunar calendar for the people of the jungle.

Great, but We’re in Space – How Do I Calendar Now?

There are a lot of different ways you can approach this. For instance, does your story/world mostly takes place on a ship where people from earth are migrating to a new planet, at least on the ship the current Gregorian Calendar can still work out fine.

You’re going to start to run into issues when people are colonizing different planets with different seasons, different lengths of days and different lunar schedules.

It’s going to get even more difficult when you run into other races and beings within the universe whose biology is adapted to different lengths of time and different seasons.

One way to approach this is to adapt a fictional system to suit your needs and fit into your own unique space. Stardate, for example, was the system used by “Star Trek.” Stardate still follows a Julian date system in some respects but uses numbers instead of names.

The decimal in the Stardate represents a tenth of the day. For example, XXXX.5 would represent noon or the middle of the day. The trouble with the Stardate system is that it has been changed and refined over time. What you should take away from this is the elements of logic at work; use that or any other fictional system chosen as a starting point for inventing your own interplanetary calendar system.

Another thing to keep in mind is that calendars throughout history have never perfectly aligned when trying to convert from one system to another (mainly for years). Think of the “Bible’s” 6k-8k history of human kind vs. the scientific calculation. There’s quite a number of missing years between the two calendars. Therefore, you could have an interplanetary system based on the scientific theory of how old your explored universe is, and then you have an alien race whose documented/known history goes back only 12000 years. There is nothing wrong with that; in fact, that’s absolutely perfect.

When coming up with your own interplanetary system, consider the races involved in its formation. Consider the races’ preexisting calendars. Think about the theorized age of the universe in order to get an idea of the span of time you need to work with.

For times of days, make your planetary days easily convertible, base 10. One planet might have a 40-hour day, while another has a 20-hour day. Thankfully, these are both divisible by 10, and dividing that in half is simple math too.

You now have to focus on the makeup of your people. If you have a planetary race that has a 15-hour day, and another that has a 40-hour day, you now run into the issue of date drift where one planet’s day might drift into a different date within your standard. Here are two things to keep in mind about date drift: first, if you’re building this world for your story, the science and complications of this are probably not going to play heavily into the story; keep with your own logic in your time/date system, and don’t overburden the reader with too much detail. Second, as long as you can easily convert, the date drift issue isn’t going to be a huge problem. Just like you have to convert times for time zones, you can do the same thing for your interplanetary system. And again, much of this is not even going to come up for the reader. Only present it when necessary.

Beware when it comes to an interplanetary time/date system that includes months, years, centuries and millennia because the universe is very old, which means you’re going to have a long string of numbers. You’ll want to devise a system to help ease this. You could use a notation system like scientific notation (which will confuse the reader) or have a colon system with a base 10 or base 100 in certain spaces.

Keep your chosen system internal – your own internal logic. It is only necessary for your audience to understand if the situation arises, and in some cases, it’s better to navigate away from your internal system and use something a little more earthly for your audience to follow.

Coming Up with a Calendar on a Multi-Moon World

Oof! Coming up with an interplanetary calendar system might seem like a tough task, what about the complexities of a world with multiple moons?

You might have already tried this and in your research of moon phases come up with a few major obstacles when planning a multi-moon calendar. Right away, the moon phases of one moon will not line up with the phases of the other. Add more moons and you’re going to have a mess.

There are a few ways to solve the problem. First, use a solar calendar instead. They make more sense in places on your world that deal with real seasons.

The second option is going to make sense if most of your people reside in equatorial locations where seasons aren’t going to affect them as much. In this case, pick the prominent moon and set your calendar to it. Perhaps your world has 12 moons and each represent a goddess; if you think about other culture groups in history, there are usually supreme gods/goddesses and less supreme. Apply that concept to your world, and pick the moon named for the supreme goddess as your calendar moon.

Do not, do not, do not, do not try to come up with a way to employ all moons’ phases into a singular calendar. This is going to waste your time and confuse the readers.

Coming Up with a Calendar on a Multi-Sun World

First, it is possible to have a Tatooine type planet that orbits two suns. And it’s a really cool idea. The question is, how do you invent calendar system based on the two stars?

Remember to think logically, and try creating from a bottom-up perspective rather than top-down. That is to say, think about the people residing on the planet and where on the planet they reside. Will seasons affect them? Will the world only have a single season? How is the rotation around the two suns going to impact seasons? Are there even months at all or just a number of days in a revolution?

Now you can start to think about the bigger issues: how long is a single season? What happens when the two suns align? Are there major seasons and subseasons as a result? How long is a revolution? Does one sun have more impact on seasons than the other (one sun being much farther away than the other).

Now construct your calendar system based on the length of a full revolution around both and the seasonal breakdown during the full year. For some guidance on how this might work, read more about the Kepler-16b planet that orbits 2 suns. According to the article, it orbits both stars every 229 days. When coming up with how long a revolution around both suns will take, try to make it easier on yourself by making it close to earth’s or Mars’s system.

Naming the Months

Naming your months could be very easy or very difficult depending on your world and your world’s people. There are a few ways you can approach this if you’re struggling to come up with names for your months:

Examples of Naming Concepts Suited for Fantasy

• Random – come up with pretty words that sound nice
• Gods – a system based on gods and lore in your world
• Kings – the first kings/legendary are immortalized in your months
• Seasons – come up with a conlang for your seasons and then divide them into thirds. For example, fall, winter, spring and summer are now Taun, Waun, Vaun and Aun And first, middle and last are init, equit and temit respectively. High winter would be a combination of the describing words: Waunequit. Early summer would be Auninit. Late spring, our May, would be Vauntemit.

Sci-fi is different. In some cases, the systems above could work well. However, for an interplanetary calendar, a numeric system could work better. Depending on the story, naming months for the solar planets could be a good concept, or naming months for famed ships.

No matter what type of world you’re building, there are three major tips when inventing months: first, keep your system logical and clear. Second, make it easy for your audience to follow. Third, keep your words easy to read and easy for your audience to pronounce.

When using conlang plus logic, it will help to surround the conlang with context so the reader can pick up on meaning. When in doubt, leave months out and stick to the seasons. Calling the time of year late spring rather than Vauntemit is going to be much easier for your audience to grasp.

For more tips on coming up with names within your world, check out our ultimate guide on name creation for worldbuilders.

The Biggest Takeaways When Creating Your Calendar

This article has quite a lot of information to help you create a calendar for your world. Some of it might be useful. You might still have some questions. So here are some final and very important notes of advice:

• Always, always, always consider your audience. Do not overcomplicate things and make it hard for them. More of your calendar system is for internal reference only.
• Always, always, always consider your world’s people. Create a system logical to their culture, needs and history.
• Research calendars through history and why they suited a particular group of people during a particular time.
• Research standardize calendars, especially during war or in military use.
• Keep science in mind and make your system follow real science.
• Don’t overcomplicate the math by coming up with a system too far from earth’s. It’s easier to stick with a 12-month, four-season system. Once you perfect your calendar creation, you can change the numbers.
• Once again, do not confuse your audience! Keep the math simple, the science real, the names easy, and most of the information internal. Don’t reference the months unless it is absolutely important to the story!