Every writer will give you a different take on this, but here’s mine:
If you write some fiction that takes place in the real world—be it the past, present, or not-too-distant future—both you and your readers get to take a lot for granted. There are 24 hours in a day, four seasons in a year, one moon in orbit, and when you refer to Ancient Egypt, readers will rightfully think of sand, pyramids, anthropomorphic gods, and maybe mummies. If a character pulls out a gun, the reader understands the concept, even if they don’t know the physics of firearms. So you don’t have to waste any time educating the reader about anything new. You can hit the ground running. I suppose it could be reasoned that this is why mystery, romance, and non-genre fiction get the most sales. There’s less work for both author and reader with "real world" fiction. But nah, I think their popularity probably has more to do with the zeitgeist of modern society.
With fantasy—or outlandish science fiction—storytelling is quite a bit more work . . . if you don’t want to cheat.
Why do I say cheating? In Star Wars, seeing two suns in the sky on Luke’s home planet of Tatooine might suggest an unusual daylight scheme. Are there, in fact, 24 hours in a day? (If so, then the circumference of Tatooine also happens to be 24,901.55 miles; if not, then it spins faster or slower to compensate to make a full rotation in 24 hours.) Do day and night have the same ratio as on Earth? Is Tatooine’s cycle around one or both of its suns the same as ours? If not, then years don’t mean the same thing. How many years old is Luke, then? Supposedly he's 19 years old in the first film. Now, is that in Earth years or Tatooine years? George Lucas didn’t worry about stuff like this because it wasn’t the point of his story (and might even have detracted from the story if he had).
But readers who appreciate immersive science fiction might demand to know such things. Such readers may well have chased down those answers in the Star Wars novels. Or maybe those in charge of the Star Wars Expanded Universe are content to gloss over those details? (Which would mean that Tatooine, Alderaan, and Coruscant are all the same size or in the same sort of orbits.) I wouldn’t know.
I may be picking on Star Wars, but it’s true for most other fantasy settings. Most of them feature worlds which are conveniently similar to ours, so that the authors don’t have to worry about such details.
With Middle-earth, Tolkien didn’t need to worry about this problem much because Middle-earth was Earth all along (and shared the same night sky, moon, and presumably the same solar system), or at least an alternate Earth. He told us only what needed telling, and if something was quite different about his setting than our understanding of Earth then he did address it. Usually. Magic, goblins, Elves, Wizards, orcs . . . these things he fleshed out quite extensively so that you do understand, more or less, where they fit into the big picture.
But what about if you’re creating a new world of your own, as I am, and you don’t want to take the easy way out? Then you’ve got physical, cultural, and environmental factors to flesh out and make clear to the reader so that they can follow the plotline. If you don’t address them, discerning readers will eventually wonder . . .
- How many hours in a day?
- How many days in a month? In a year?
- Are there 7 days a week? If so, why? Isn’t our calendar astrology- or Bible-based, and therefore quite specific to Earth?
- Is the world round? Do the people know it?
- Is the character dialogue we’re reading merely being “translated” into English from some fantasy languages just for our benefit? Or am I to believe that Latin-based English is what these characters are really speaking?
For my world, the intent is to straddle the line between adaptability and singularity. I’m probably not going to go and invent some hard-to-follow system of time that will detract from the story. There are 24 hours in a day, and probably 365 days in a year. But that’s not mere happenstance. The similarities between Earth as we know it and this setting are deliberate, as will be the differences. The calendar won’t be precisely the same as you know it. But in this first book, what we call dawn doesn’t arrive until around noon. Daylight is scarce. And you'll find out why.
And the language everyone’s speaking . . . is it English? Yes and no. Wait and see!