If you are writing a fantasy or a science fiction, how do you come up with the year(s) its all taking place? The first example that comes to mind is Tolkien: "In Tolkien's cosmology, Arda (the Earth) is at first created without the Sun and Moon to illumine it, and its earliest history is measured in Valian Years (V.Y.). After the creation of the Trees of the Valar, a new tally of Years of the Trees is begun in V.Y. 3501. At about V.Y. 4550, the First Age of the Children of Ilúvatar begins with the Awakening of the Elves." (Wikipedia) How do you come up with yours?
Well, I tend to look at my plot. Originally my story took place on an alternative Earth but then I realized it was so much like Earth and that is disliked doing the science of building a planet that I just made the planet earth in the future. Cut my work down a lot. Now I don't have to create my own names for places and characters - I'm not a huge fan of world-building. I usually figure out my plot and character arc then fill in the rest.
You can create a timeline AS you write. In other words, it's not so much a framework to build your story around, as a record of what you've done. So you don't forget how old your character is when certain things happen, etc. I use real world history, so when I'm writing I try to keep track of what else might be going on at the time my story is taking place. My setting is the late 19th century, so there would be news and awareness of things happening even far away. The news might take a little time to travel, compared to the internet and daily worldwide news bulletins, but it's not as if my characters live isolated lives. Just like today, news does affect our lives to some extent. As do things like laws, weather conditions, travel conditions, etc. And inventions/technology as well.
For example, the novel I'm currently writing is about two civilizations meeting. In the prologue, I inform the reader that the two civilizations' first contact became later the starting point of their common calendar. That way, all dates are relative to the moment of that first contact, and how that relates to the timeline of our Earth is left undetermined. I looked at major events and asked myself how much time could there have plausibly been between them (for example, between the first contact and the beginning of the great war, between the beginning of the great war and the beginning of peace negotiations, etc). After that, I put the minor events into their more or less proper places. If I would write about something happening on our Earth in the future, I would just place it in a safe distance from our lifetime. I would feel silly if I wrote that something's going to happen in 2030 and then the real world anno 2030 would turn up very different from what I had written.
My timeline is tied to the story of my characters. Specific events have to have taken place either influenced by my characters at certain points in time, or I needed them to set the border conditions for events that my characters set in motion. So there's no way I could make up all of this as I went. I first figure out the setting of my key scenes. These settings give me the conditions of the outside world at these points. Then I make up a timeline leading from and to them, so that conditions at a) can conceivably end up morphing into b) . That's it. *whispers* but there's a helluva lot of research involved.
I go by the age of my characters. So, for example, I don't have many specific years mentioned in the story but know it takes place several decades in the future. However, I do mention my character's ages multiple times throughout the story, and that should tell the reader how long it's been since something happened and with the age also comes different traits. My story is driven primarily, and at some points exclusively by the characters.
So a character's age and location would suffice rather than a specific year? The first 18 pages of a WIP is a specific event that basically launches the rest of the plot line. After the 18 pages or so, it leaps 17-18 years later in a completely different location. It felt cheesy to me to write "17 years later..."
You can get a little creative with saying "17 years later." For example something like this could work... After almost two decades of heavy drinking, Mike still couldn't shake his alien encounter. It very quickly let's readers know time has passed and that Mike hasn't been doing too well since the last place you left readers in your story.
I need to go back to some ancient Roman writings and see how they reference time pre-Julian and pre-Gregorian calendars. Even today there is no universal ýear'. It really tripped me up in Nepal when they kept referencing the year as 2075 (it's 2076 now)
That's what I do. I'm intentionally vague about that sort of thing precisely so I can be as creative as I want on my own time. In fact, I'm not sure I'll ever be precise about dates. Even in long series or lengthy trilogies it just doesn't seem all that important. The Hobbit was written before Tolkien had made up his mind about many of the details of the Silmarillion. And actually, he didn't add The Hobbit to his larger universe's growing history until starting TLTR! And both books are terribly vague about time. When I do create timelines, I always start with what I want the oldest story to be. Then I make up numbers, typically bigger ones assuming my story isn't opening a new age or something. Gotta decide how the main people groups tell time, too. How do they group large numbers of years and what do they call them? Do they name specific ages or eras? That sort of thing.