A fantasy world can come in all shapes and sizes, and all kinds of eras. Sometimes it’s no more than a few countries surrounded by vague space that could well be marked Here Be Dragons (though if that’s meant literally, they’re probably part of the story). Sometimes it’s a continent surrounded by an ocean of unknown extent, or even a couple of continents. Sometimes it’s an entire world, whether that world is spherical, flat or banana-shaped.
In the same way, it could resemble a primitive civilisation, the classical world, the mediaeval age (though far fewer worlds are mediaeval than is often claimed), the Renaissance, the age of steam, or even a modern-style world. Or it could be based on some phase of a non-Eurocentric culture, or perhaps it’s a civilisation like nothing ever seen in our history.
Nevertheless, there’s one element the great majority of fantasy worlds are lacking in: time.
Hang on, what? Aren’t they usually rich with the history of millennia, and doesn’t that tend to impact directly on the plot?
Yes, but history isn’t the same as time. In most worlds, the history remains no more than history — looked back on, but never visited. This doesn’t necessarily stop them being wonderful creations. Classic worlds like LeGuin’s Earthsea and Martin’s Westeros have rich histories to back up their rich presents, but if we ever visit those past times at all (as we do in some of LeGuin’s short stories, for instance) it’s strictly in the mode of Tales of the Old Days.
There are certainly exceptions. Tolkien covers thousands of years, but the point here is that his portrayal of Arda is a mythical past for our own world, from the supposed point of view of a modern scholar translating texts from different eras. Howard takes a similar approach, where Conan, Kull and other characters live at different stages of his imaginary prehistorical history for our world.
It's rare* for entirely secondary worlds to be presented with an intact fourth dimension, in which all periods are equal and there’s no fixed concept of “the present”.
The Chronicles of Narnia are perhaps an exception, showing the entire life of the Narnian world from creation to doomsday, but that’s due to the nature of the portals. It’s being seen from a perspective outside the history portrayed, just as Tolkien’s Arda is seen. Donaldson’s Thomas Covenant books use a similar approach.
In most secondary worlds, though, the past is no more than the far-off hills in the background of the picture, while the future is a non-existent place outside the frame, unless it’s just a generation in the future when the account you’re reading is being written (which therefore becomes more accurately “the present”).
What I try to achieve is an approach that’s not so much equivalent to a two-dimensional picture as a virtual tour, where you can not only pan around but also go over into the distance, turn 180 degrees and look at what previously lay behind you — ie the future.
I write about characters and events in “traditional” fantasy cultures, and then about modern-style eras where the same people are the legendary heroes of old. I write stories set in all stages of civilisation, which look back to those that were the “present” last time, and foreshadow what will be the “present” next time. I’ve covered everything from the neolithic to the computer age (and a glimpse into a futuristic age long after) — and no point is the absolute present. It’s all in flux.
The absence of the present is perhaps the key point, and it’s more common in SF future histories than in fantasy secondary worlds. Asimov treated all eras as equal, from the early development of robots to the supremacy of the Foundation, while the various iterations of Star Trek are each seen embedded in their own present. And it’s perhaps significant that the Pern series lost its sense of “the present” just at the point that it was becoming more openly future history, rather than a secondary world.
Perhaps it’s easier with future history, where we can see time stretching out ahead of us without end. On the other hand, if a secondary world has no connection with our own, why should it have a fixed present?
Of course, if you’re just using the world for one story, whether told in a single book or a long series, it makes sense to relate to the time that story’s taking place. But a well-made world almost certainly has more than one story in it, and it would be awesome to see more fantasy authors embracing the full implication of that and writing in four dimensions, not just three.
Those hills don’t have to stay in the background of the picture. You’re free to go and explore them, as well as to break out from the frame into times to come.
* I’m certainly not widely-read enough in secondary-world fantasy to say with authority it doesn’t happen, and I fully expect to be told about exceptions in the comments. I’d still maintain it’s rare, though.