When I encounter the same misused fact three times in a month, I conclude it is time to spread the word about it. Ussher gets mentioned with a sneer in college introductory texts, science writing, and popular anti-religious works. It is Ussher, a 17thC cleric, who set the year of creation at 4004 BC. October 23, to be exact. This provides grounds for general hilarity and mirth, of course.
As a mere curiosity, it would still be an unfair characterization, because Ussher did not merely try to count up all the ages and years of The Begats in Genesis and just whip out an answer. He was not quite the first to try and harmonize the biblical years with other known historical texts, but he was the best of the first. His goal was not to set the day of creation - that was a byproduct - but to try and set the biblical events as exactly as possible among the chronologies recorded elsewhere. Sciences of archaeology, geology, and biology as we know them did not exist then. For all anyone knew then, there was only history to study from. Written history goes back to about 3500BC. There were some earlier markings with meaning, but nothing that recorded history as we think of it. He worked with what he had, and did a darn fine job of correlating Persian, Egyptian, and other historical documents with biblical accounts. Other cultures with writing actually tended to agree with the general time of 4000 BC. Archbishop Ussher's chronology for Abraham or Saul is still considered pretty good.
But there is a further unfairness beyond the usual chronocentrism common to our era, which regards all who came before us as fools. Modern references to Ussher usually imply that the Christian churches embraced this accounting wholeheartedly and taught it as scientific fact up until oh, last Tuesday. While you can certainly still find Christians who believe in a 6000 year-old earth, you don't find many. Ussher's exact dates stopped being entered marginally in the AV around 1800, and the 4000 BC idea was abandoned shortly after. Well before Darwin, or theories of geological eras, in fact.
I am certainly no six-day creationist, but for those intimidated by all the National Geographic articles and science textbooks which refer lightly to human beings existing 2.5 million years ago, remember that it matters greatly what you use as your definition of "human being." Language doesn't show up until around 100,000 years ago - that's the last 4% of 2.5 mil - art about 50,000 years ago, and agriculture less than 10,000 years ago. Something interesting and unusual did start happening about six or seven thousand years ago. Which is why writing, cities, animal domestication, irrigation and a bunch of other hey-that's-like-us behaviors started shortly after.
So everyone stop picking on Jimmy Ussher.