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I really think since you already have a fairly accurate timeline to work with, that you should run with that.  I really don't think cover wise that you need to be any more in depth than a simple four digit year.  My suggestion would be to put the exact date on the cover page or something like that and then maybe refer them to a web based, interactive timeline so they can visually see the eras and how each story fits in to them.  The following link is to a free online timeline maker, I'm sure there are many other this is just for an example.