In the lower right part of your screen there is a bookcase. If you click on the book to the right it will open up to a page with something like "author's foreword" (or just click "H"). If you keep turning the pages you will eventually come to a page "war" where you can find a date. Add four months to that date, and you know when the messenger will turn up.

Or you can just keep doing quests and training and visit Harmondale sometimes, sooner or later he'll be there.