Date of creation
Calculating the date of creation has exercised many minds for hundreds of years.
Basis of calculation
The Bible contains chronogenealogies from Adam to Abraham, listing the age at which each person in the genealogy gave birth to the next person in the list, thus allowing by simple addition a determination of how many years passed between creation and Abraham. There are other chronological indications also, allowing that calculation to be extended into the times of the kings, when the dates can be correlated with other events in history for which the absolute date is known. By this means, in theory, one can calculate the date of creation.
There are, however, some uncertainties in this. For example, Genesis lists the age of Terah when the first of his sons was born, but was Abraham this son, or the second or third son? Different researchers have proposed different answers to this and similar problems, but none of these problems change the result by a very significant amount.
An objection that has often been raised is whether the chronogenealogies were intended to be used this way, or whether, for example, generations have been missed. Henry Morris, a pioneer of modern creationism, allowed for the possibility of gaps in the genealogies, but claimed that it was unreasonable to stretch the date back further than about 10,000 B.C. In more recent times, however, creationists have concluded that there is no reason to think that there are gaps, and this view is supported by the consensus of the experts, according to James Barr, then regius professor of Hebrew at Oxford University, and not a young-Earth creationist (he also wrote a book attacking biblical inerrancy), who wrote in 1984: "… probably, so far as I know, there is no professor of Hebrew or Old Testament at any world-class university who does not believe that the writer(s) of Genesis 1–11 intended to convey to their readers the ideas that: … the figures contained in the Genesis genealogies provided by simple addition a chronology from the beginning of the world up to later stages in the biblical story." 
Old-earth creationists do not accept this date, proposing that there may be gaps in the genealogies.
The best known date of creation is the one calculated by Archbishop James Ussher in the 17th century - namely 6:00 p.m. Saturday, October 23, 4004 BC. Ussher calculated the date of Creation by the following means:
- He accepted the date of the death of Nebuchadnezzar II of Babylon at 562 BC.
- He then assumed that Evil-Merodach began to reign in that year.
- King Jehoiachin received a pension from Evil-Merodach beginning in this year, and that he was taken captive 37 years before then, or in 599 BC. (The final Fall of Jerusalem occurred eleven years later; hence Ussher places the Fall of Jerusalem in 588 BC, not 586 BC as most secular archaeologists assume.)
- From that anchor point, Ussher worked backward through the king lists of the Divided Kingdoms Northern and Southern. See I and II Kings.
- He worked backward further to set the dates-of-reign of King Solomon, and calculated the Exodus at 480 years earlier than the groundbreaking of the Temple, which was in Solomon's fourth year. This fell in 1012 BC, and so Ussher fixed the Exodus at 1491 BC.
- Based on his interpretation of Galatians 3:17 , Ussher then fixed the date of the entry of Abraham into Canaan. This was in 1921 BC.
- Ussher here made a key assumption that is in great dispute. We read that Abraham was 75 years old when he embarked into Canaan. We also read that Terah was 70 years old when he "begat" Abraham, Nahor (the younger), and Haran. Ussher's assumption, which added another sixty years to the reckoning of Creation, was this: that Abraham did not embark on his own until after Terah had died at the age of 205. This would mean that Terah was actually 130 years old, not 70, when Abraham was born--and presumably that Nahor the Younger or Haran was born when Terah was 70. Ussher's sole warrant for this assumption is that the Bible describes Abraham's departure after it describes Terah's "death." But Terah's "death" might be spiritual rather than physical, in that Terah had originally intended to take all his family out of Ur of the Chaldees and into Canaan, but forgot his purpose and grew too accustomed to worldly enticements in the country of Haran. If that is the case, then Abraham might have departed when Terah was still alive--which is what the inventors of the present Hebrew calendar assumed.
- Ussher then backtracked the pedigree of Abraham to Arphaxad, born to Shem two years after the Great Flood. He therefore concluded that the Great Flood happened in 2349 BC.
- Finally, Ussher backtracked the genealogy of Shem to Adam. He further concluded that Creation must have occurred during the Autumnal Equinox, which in fact is the favorite start of many of the world's calendars, ancient and modern. He also assumed that the ancient Hebrews did not attempt to synchronize their months with the moon until after their exile into Babylonia. He thus calculated the date of creation at October 23, 4004 BC according to the Julian calendar.
James Ussher's calculation was the best-sourced calculation in all of Christendom at the time. Ussher also spoke with authority, and from a position of authority. For those reasons, his dates for various Biblical events appeared in the margins of King James and other Bibles for centuries, until the last quarter of the twentieth century, when publishers abandoned this practice. Sir Isaac Newton defended Ussher's date.
Another to calculate the date of creation was Johannes Kepler, who worked it out to be 3992 B.C.
Such calculations in fact date from at least the Middle Ages, and offer a range of dates from around 5000 to 4000 BC. Judaic tradition reckons the date of creation at 3760 BC. Another calculation, beginning with the date of the destruction of Jerusalem known from secular history and working backwards, arrives at 4163 BC. 
Based on geological dating methods (which include certain assumptions which are questioned by creation scientists), secular scientific research dates the formation of the Earth to around 4.5 billion (4.5 * 109) years ago. And according to the Big Bang theory, the universe began even earlier, about 13.7 billion years ago.
- Quoted in Biblical chronogenealogies, by Jonathan Sarfati.
- Are There Gaps in the Biblical Genealogies?, by William Henry Green.
- James Ussher, The Annals of the World, Larry Pierce, ed., Green Forest, AR: Master Books, 2003 (ISBN 0890513600), pgh. 1ff.
- James Ussher, op. cit., pgh. 891
- James Ussher, op. cit., pgh. 892
- II_Kings 25:27-30
- I_Kings 6:1
- Genesis 12:4
- Genesis 11:26
- Genesis 11:32
- Genesis 11
- Genesis 5
- Cited in Sarfati, Jonathan, Comets—portents of doom or indicators of youth?, Creation 25(3):36–40, June 2003.
- http://web.newsguy.com/rubyredinger/age.html Biblical age of the Earth
- For example, see Batten, Don, et. al., The Creation Answers Book, chapter 4, 2007)
- USGS, Geologic Time: Age of the Earth. Accessed April 18, 2007.