A writer can simply write about the present, of course, about current events, about the world they see around them. But they can also choose to write about something different. They can write about the past, the future, or something that has never existed, and likely never will.
Yet these are always filtered through the present. Indeed, they are filtered through the writer's consciousness, and this is ultimately a creation of the present moment. Words come from a specific mind, and a specific moment in the history of that mind, and that particularity in time is very important. A word written down yesterday might still seem right, still seem like you... at least most of them. Already, though, you start to wonder: why did I choose that word? That doesn't seem right...
And words chosen a year ago? A decade ago? They might seem utterly unfamiliar, with only a vague sense of deja vu connecting you to these oddities. You shake your head, you wonder about different words, better words... words of the mind's inescapable present.
It does not always matter what you write about. You can write about ancient history, but in some sense you're always writing about the present, writing from the present. These times are seen through the lens of now.
Sometimes this is obvious, with stories full of modern characters dressed in period costumes. And, obviously, deeper imaginative connections can be attempted, to see into a distant world, to sincerely feel and experience the possibility of such a life, such an existence. But we can never completely escape who we are--creatures of the present. The actual experiences of those people are gone, vanishing into that vast, opaque river that is time. We can research, we can delve for fact, for the right words, for accuracy... and yet our understanding of people is always a little anachronistic. Our understanding of people will always come partly from our own experience, from the people we've seen, the people we know, the people we've observed. We are endlessly filtering and transforming the world around us. Sometimes the transformations are small, and sometimes the transformations are large, visions of a seemingly new world. But, beneath the surface, there are always tethers to the present. We cannot escape our own subjectivity.
Indeed, why are such historical and futuristic subjects chosen? Likely because there is some element of conflict, of theme, with a deep connection to the writer's present, to the world around them, to the immediate sensations of their own psyche. There's a need. Something being sought, or desired, or felt. And this can impel exploration, and may send the mind racing backward, or forward, or into the fantastical unknown.
Or, of course, we can simply choose to write about the world around us, as we know it and see it. The need, here, is very immediate. Something is seen, or experienced, and it impels a question that needs some answer, with the written words as an attempt at just such an answer. And sometimes the answer isn't found, or the answer is a poor one. But there's no necessity for good writing. Everyone can't write brilliantly, and the quality of the expression of that answer will usually mean much more to an audience than to a writer (and great writing is usually going to be about the law of averages - to have one great piece of writing on a subject, many people will likely have to attempt it). To the writer, perhaps, all that really matters is the attempt. To answer the question, or to satisfy that strange need.
Perhaps the writing will be great. Odds are it won't. But the writing itself is important, the present act. Memoir, I think, is the clearest example of this. It is the present mind's attempt to answer questions about its own past, to bridge the gap between then and now, but the story is always filtered through the immediate consciousness in the now. Only that present mind exists (for the merest of instants), and its job is to try and excavate the past, piecing together something that no longer exists, a personal experiment in human paleontology.
