The following article is adapted from a longer series of posts that I originally wrote for the Historical Writing section of Compuserve's Literary Forum in 1996. (The Literary Forum still exists under the name "Books and Writers Community", but its structure has radically changed, so most of the old sections are defunct.) I wrote the original in response to a skeptic (that's the polite word) who proposed the theory that conventional historians had grossly mistaken the chronology of both the ancient world and the Middle Ages. In the original piece, I tried to explain how historians piece together different facts to arrive at a chronology. Along the way, I described different historical calendars and the origin of the western calendar. In the following, I have extracted that portion of the earlier article that bears on the origins and history of the western calendar. As it has only been lightly adapted, the work retains something of its original focus on the justification for the traditional chronology used by historians, but it should stand alone. If you would like to see the original work, you can find it here. The original threadded conversation that inspired this outburst is still available Books and Writers library.