The age of the Earth is 4.54 ± 0.05 billion years (4.54 × 109 years ± 1%).
-Wikipedia, more on the story here
As we first learned from Frederic March in Inherit the Wind, Archbishop James Ussher calculated, in the mid-1600's, that the earth was approximately 4,000 years old. It is an interesting story. Ussher's wiki is here. A few excerpts here:
"Ussher now concentrated on his research and writing and returned to the study of chronology and the church fathers. After a 1647 work on the origin of the Creeds, Ussher published a treatise on the calendar in 1648. This was a warm-up for his most famous work, the Annales veteris testamenti, a prima mundi origine deducti("Annals of the Old Testament, deduced from the first origins of the world"), which appeared in 1650, and its continuation, Annalium pars postierior, published in 1654. In this work, he calculated the date of the Creation to have been nightfall preceding 23 October 4004 BC."
"Ussher's work is sometimes associated with Young Earth Creationism, which holds that the universe was created, not billions of years ago, but thousands. But while calculating the date of the Creation is today considered a controversial activity, in Ussher's time such a calculation was still regarded as an important task, one previously attempted by many Post-Reformation scholars, such as Joseph Justus Scaliger and physicist Isaac Newton."
"Ussher's chronology represented a considerable feat of scholarship: it demanded great depth of learning in what was then known of ancient history, including the rise of the Persians, Greeks and Romans, as well as expertise in the Bible, biblical languages, astronomy, ancient calendars and chronology. Ussher's account of historical events for which he had multiple sources other than the Bible is usually in close agreement with modern accounts – for example, he placed the death of Alexander in 323 BC and that of Julius Caesar in 44 BC."