This is a matter that I've considered to great lengths. I'm building a fantasy world and writing novels within that world. With fantasy, there are a lot of things to consider, the primary one being magic. What sort of magic is it? How does it work? How powerful or common is it?
There are other things to consider, like whether or not humans are the only race, or do you throw more in? Many writers add elves, dwarves and other Tolkienesque style races in. At first I was going to add those, but decided to go with all human instead. That changed when I added the Rojuun as a race. But in doing so, I tried to make them largely unique, not following any of the normal fantasy stereotypes.
Plants and animals have to be considered. If you make everything in your world brand new, you use up all the words describing those things. I decided to keep a lot of the same trees and animals that are found on Earth. I add extra flowers, but keep a lot of what's on Earth too. The way I make it a fantasy world is by adjusting things, and adding supernatural creatures. I have emo bunnies (surprise, surprise) carnivorous fairies, dragons, dryads, Mother Trees and things like that. Also, the flowers are a great deal more powerful and sentient, which came about with the creation of Liselle as the Flower Child.
Currency, architecture, level of technology and how much magic replaces science are all important considerations too, but this blog isn't about those things. It's about time.
These are many questions I've asked myself: how many months should a year have, how is the month measured, especially since I have two moons, how many weeks and days in the month? How many hours in the day? Do they even have hours? What about minutes and seconds? How do they tell time? How did people measure time before technology? Does anybody really know what time it is?
Deciding what elements to add
At first I didn't want to stop writing to spend who knows how many hours figuring out a system. I did briefly look at other books to see how they did it, but nothing jumped out at me that I was desperate to use. At the time, I stuck with normal time because I wasn't about to interrupt the flow of words that was coming. Writing can be very hard sometimes, especially when you're first starting. I didn't succeed until I set aside all the distractions and just plowed through the first draft of the story.
The time came to make a decision and I finally decided to just stick primarily with standard measurements, though I made some adjustments. There are twelve months in the year, BUT I don't actually talk about it in the story. I've written three novels and have started the fourth in the world of Ryallon, and not once have I said what month it is. All of my descriptions have been by season instead.
In the first trilogy, Book 1 started in early spring and ran through to early summer. Book 2 went from early summer to late summer. Book 3 went from early fall to late fall. I eventually had to do a timeline of everything that happened to make sure I wasn't messing it up. I also had to figure out the phases of the two moons in the world. Luckily I didn't seem to mess anything up too badly.
The book I'm currently writing occurs years earlier than the other trilogy. Originally I was going to start it in the middle of summer, but as I was writing I realized that it was snowing, so I switched the season to late fall. Writing is like that sometimes; the scene or the characters decide what they're doing without really telling you. It happens with weather too. I've learned to go with the flow. The writing is much more natural and fun that way.
The months have four weeks of seven days, making them twenty-eight days long. I haven't told the reader that either, but use the knowledge for myself. I haven't named any of the months or days either and have no clue what they're going to be called. Part of the problem is that when I do, I have to tell the readers and explain why everything is named that way. It's more important to me to describe things like magic and location as well as tell the story. Actually, the story is the most important part, obviously.
I figure different civilizations in the world will have different calendars too. Humanity has been that way, why not a fantasy world? But I can leave that until necessary or interesting.
Then we get to hours and minutes. I do not define seconds, instead calling them moments. Seconds seem like a modern concept and they don't really fit in my image. I've decided to keep hours and minutes because the reader can relate to them. When writing, especially sci-fi and fantasy, it's important to give the reader stable information that they can relate to. It helps them come to terms with the fantastic.
I'm finding that sometimes the information that's not told to the reader is just as fascinating as what is told. The reader will fill in the missing information with what they already know. As I write more, I learn what is and isn't important to describe. It's really interesting to me, honestly.
So, the final result is that while I have years, months, weeks, hours, minutes and moments, to the reader, I only define seasons and times of day: morning, afternoon, evening, sunset, sunrise, early night, late night, early morning, noon, before sunrise . . . etc. At no time have I told them what year it is, what month it is, what day it is or exactly what time it is on the clock. When I realized this, I found it amazing. I like it too.
My characters know what time it is though. I've had them tell each other that they would meet in an hour or that something would happen in a few minutes. The words were necessary for them to communicate to each other. Even with that, they were talking to each other, not the reader. It's a subtle distinction.
In finishing, I'd like to state that these are not rules and guidelines on how to do things. Every writer is different and finds systems that work for them. I talk about how I do things so that other writers can see how someone else does things and maybe get an idea. I also do it so that my readers can get insight as what I am thinking when I write. (I caution them against delving to deeply into my thoughts because it's scary in there)