In our modern world, we’re used to the Gregorian calendar being accepted worldwide. This wasn’t always true and in some places it’s not official and is only used when trying to correlate dates between different countries. We could invent a datebook for every kingdom, but that’s a lot of work for little gain. A universal calendar, meaning one that’s acknowledged across a planet, has both advantages and disadvantages.
In stories, we might have little reason to mention the year, but this depends on our needs and the world’s technological level. A fantasy setting might be less concerned with information in general, not to mention date, but some SF frequently mentions this, such as the well-known “star date” for a captain’s log entry in Star Trek. What’s important is that, whether we mention dates or not, we need a universal calendar for our notes even if not used by our characters. Otherwise, we can’t reconcile two differing calendars and understand when events occurred. There’s no way to tell that year 734 D.C. in Kingdom X is thirty years later than year 343 O.E. in Kingdom Y.
Each sovereign power may have its own time measurement. Year one will be an important event, such as the kingdom’s founding or the life and death of someone important. There could be a technology that shapes life, or a supernatural event or discovery. It should be something that resonates with society or which the power wants to champion, such as the first ruler in a dynasty. While we’re free to invent these internal calendars, we need a universal one.
Whether our universal calendar is recognized on the planet is another matter. For this to happen, there must be a globally accepted event. On Earth, we use the birth of Christ, with years before that counting down like a timer and the years after counting up. However, this scheme wasn’t recognized for hundreds of years and didn’t become standard for hundreds more. Christianity slowly spread, and with it, the calendar. For an invented world, we might need something like the birth of magic or a new technology to occur and slowly spread, too. A swifter way is a cataclysm, which is especially viable in post-apocalyptic fiction and which will quickly dominate minds and ways of life. A mass exodus from a planet might be, too. We need to think of an event that most people in the world will think is a big deal.
If we want our story to be directly impacted by the event, by which I mean the characters and world are still recovering from it, the event should be recent, within the last few hundred years. Otherwise, place the event farther in the past. The invention of a technology might impact modern life, but the moment of that invention doesn’t have to be recent.
Deciding no universal calendar exists is also fine, but be sure to choose one for your private notes if you’re building a world intended to be used a long time, and which has multiple sovereign powers with differing calendars. If you can’t decide, then go with a believed creation date rather than the timespan required by evolution. It took millions of years for life to evolve on Earth, but according to some religions, God put us here less than ten thousand years ago. We can also decide that the first civilization is a marker, as that might’ve been much closer; years longer than five digits (10,000) can seem unwieldy.
When it comes to the initials like A.D. and B.C., whether for a universal calendar or not, we’ll want a naming convention of some sense, such as B.M. and A.M., as in “Before Magic” and “After Magic.” We don’t need to use this style. Terms such as Heisei 12 in Japan means it’s the twelfth year of the current emperor, named Heisei. People might want a positive spin on the years, but in a dystopian setting, a negative sounding term can be accepted.
If we’re inventing a world for long term use or might use both hemispheres, there are some issues to consider for calendar names. On Earth, January is a winter month to some and a summer month to others, and since the word “January” has nothing to do with a season, we’re okay. But what if it was called “Snowtime” instead? That would only make sense to areas which receive snow in January. Tropical climates may never get snow regardless of hemisphere, polar areas always have snow, and some areas have no seasons.
If we want to do it anyway, regional terms can add dimension. A character from such a place can express how different things are back home, now that they’re away. On Earth, there’s no shortage of people from the northern hemisphere saying how weird it is to celebrate Christmas in summer while south of the equator. Some holidays, like Christmas, have nothing to with a season but have become associated with it.
It’s tempting to decide the year starts with spring, but this is once again only true for certain areas. For others, it would be the first day of winter or not associated with any specific season. How likely is it that everyone north of the equator likes that spring idea and everyone south of it likes the winter idea? Not very. The likely result is a different calendar, which might be local in focus.
So what do we start the year with? An event that has nothing to do with weather, climate, or seasons. In this chapter, the “Historical Event Categories” section may provide ideas.