When Moses, under the inspiration of God, compiled the account of creation in Genesis 1, he used the Hebrew word
yôm for 'day'. He combined
yôm with numbers ('first day', 'second day', 'third day', etc.) and with the words 'evening and morning', and the first time he employed it he carefully defined the meaning of
yôm (used in this way) as being one night/day cycle (
Genesis 1:5). Thereafter, throughout the Bible,
yôm used in this way always refers to a normal 24–hour day.
2,3 There is thus a
prima facie case that, when God used the word
yôm in this way, He intended to convey that the days of creation were 24 hours long.
Let us now consider what other words God could have used, if He had wanted to convey a much longer period of time than 24 hours.
....The meaning of any part of the Bible must be decided in terms of the intention of the author. In the case of Genesis, the intention of its author clearly was to write a historical account. This is shown by the way in which the Lord Jesus Christ and the Apostle Paul regarded Genesis—that is, they quoted it as being truth, not symbolic myth or parable.
5,6 It was plainly not the author's intention to convey allegorical poetry, fantasy, or myth. And so what God, through Moses, said about creation in Genesis should not be interpreted in these terms.
http://www.answersingenesis.org/creation/v19/i1/days.asp