Many books you read or have read in the past are timeless. These are the sort of books you should aim to write. Those books don’t go out of style because they were never “in style.” Their clarity of purpose fulfills us. They weren’t made to be in fashion or with the times. They were intentionally created for whatever end they once needed to serve. Marcus Aurelius produced Meditations for himself, and that purpose is evident throughout the entirety of the writing. He wasn’t worried about how to sell more copies.
If you must add timeliness to your work, make it separate from the writing’s primary purpose. Put it in a footnote. Don’t ruin timeless.