This is just about primary source in your native language, or at least the language you write in. Hmm, maybe translations, too if they are not too recent.
Because, if you wish your prose to back up the assertion that your characters do not inhabit the fields we know, it helps if the prose does not reek of here and now. Yet prose that reeks of here and now surrounds us -- not all of it, but there's very little to help us judge that if we just read here and now. Better to read from many decades or even centuries. This helps give you an ear for what in the language sounds timeless.
You may pick up some archaisms in this way, but the chief virtue is to start to recognize neologisms. You can't make your English literally timeless, but you can come closer if you make it something that would have been perfectly intelligible a century or two ago.
One thing that it helps with is weeding out the metaphors. This can be tricky because many of them in modern speech are so deeply embedded that you do not realize that "firing" an arrow is a metaphor no one would have used before they literally fired a gun. Shifting your metaphors around to something the characters can understand -- even if used by a third-person omniscient narrator -- can do much to situate your story.