Six literal days
The following is an extract from a letter written in 1984 by Professor James Barr, who was at the time Regius Professor of Hebrew at the University of Oxford. Note that Professor Barr does not claim to believe that Genesis is literally true, he is just telling us, openly and honestly, what the language means.
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 Gen 1 to 11 intended to convey to their readers the ideas that (a) creation took place in a series of six days which were the same as the days of 24 hours we now experience (b) 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 (c) Noah's flood was understood to be worldwide and extinguish all human and animal life except for those in the ark. Or, to put it negatively, the apologetic arguments which suppose the 'days' of creation to be long eras of time, the figures of years not to be chronological, and the flood to be a merely local Mesopotamian flood, are not taken seriously by any such professors, as far as I know.
According to Bishop Ussher, the six days of Creation were in 4,004 BC. This is based on a literal interpretation of the Bible, especially from the various recorded genealogies (such as those in Genesis 5, 10 and 11). This basis estimates that the Universe is about 6,000 years old. Other Young Earth Creationists say that there may have been some additional generations and extend the dates out, such that the age is more like 10,000 years.
All of the creation models are based on mankind being young (thousands of years old), other than Theistic Evolution , which is based on mankind existing for 50,000-200,000 years, the same as Atheistic Evolution. This is explained on the basis that the genealogies in Genesis are just listing some people and there are significant gaps between them.
Evidence from Archaeology and Anthropology support the view that "civilised" Homo Sapiens are young - i.e. we have only existed for thousands of years. Fossil evidence for humans using agriculture and living in settlements only goes back as far as 10,000 years. However, this dating is based on carbon dating methods, so is not necessarily accurate.
A literal interpretation generates many questions, for example: How can there be a day before the sun and moon are created on day four? However, there are plausible answers: The new creation described in Revelation 21 doesn't need the sun to shine on it, because the glory of God will be the light source "The city does not need the sun or the moon to shine on it, for the glory of God gives it light, and the Lamb is its lamp." Revelation 21:23. So it was in the beginning - God's glory provided the light on the first day, God didn't need the Sun to produce light - the light of God's glory is much greater than the Sun. Isaiah 45:7 "I form the light and create darkness, I bring prosperity and create disaster; I, the LORD, do all these things."

Further reading: