I'm currently writing a novel that takes place VERY post-humanity (genocide by biochemical warfare) to the point where the surviving human bloodlines have completely forgotten of their past and have hardly begun repopulating the otherwise undamaged world. This, of course, would leave room for quite a bit of evolution to take place. Even though most characteristics of people, places, and things are very similar to life as we currently know it, it will still be alien or fantasy-like. My issue is whether or not I should state an accurate chronological timeline, or if I should just be very vague. Like, instead of "The war happened five thousand years ago." have it be "The war happened many centuries ago." I'm just worried that my estimate will be very wrong. If I throw in what I think seems chronologically accurate for the state of evolution my world is I could possibly ruin the story for some readers who know otherwise. So what do you guys think? Should I use exact time frames, or be vague?