Simply put, the Day-Age view interprets each of the days in Genesis 1 as an age or eon. However, Genesis 1 specifically says that for each day there was evening and morning, two components of a day as we understand it today. We understand the time period of a day as one rotation of the earth allowing the sun to pass through our frame of reference once every 24 hours. To try and stretch out those days following creation of the sun (Day 4), we have to assume a speed of the earth's rotation which has not been observed nor specifically indicated in the text.
Consider that vegetation was created on Day 3, waiting much longer than a day without the sun could cause the vegetation to die. However, Genesis 1:3 does indicate that there was light prior to the sun. Revelation 21:23 describes how in the new Jerusalem there will be no need for the sun because God’s glory will light it. Exodus 13:21 describes a similar provision by God as he led Israel by night in a pillar of fire to give them light to follow. Genesis 1:2 tells us that God moved upon the waters. If this was an orbit, it would explain the evening and morning without a sun. If God had been providing the light in the early days of creation, the requirement for standard length of a day is removed and a cycle of light and dark could be greater than (or less than) 24 hours without killing the plant life. However, there are other occurances in scripture where cycles of light and dark do not correlate with a 24-hour day.
Exodus 10:22 describes the plague of darkness over Egypt as lasting three days. Here days are used to count the length of time of the darkness. Therefore, day is not described as a period of light and dark, but rather, we presume, a standard 24 hours.
In Joshua 10, the Lord caused the sun to stand still to allow Israel to defeat its enemy. Joshua 10:13 says “and hasted not to go down about a whole day.” Here a day is not counted simply as a cycle of light and dark, rather, a day is used to count how long the sun stood still. Again strengthening the concept that a Biblical day is a set number of hours.
Yet another example is at Christ’s crucifixion. There was a period of three hours of darkness in the middle of the day (Mark 15:33). Jesus was resurrected on the third day. Jesus had predicted his resurrection as the sign of Jonah, three days (Matthew 12:40). If the Biblical day was simply a cycle of light and dark, this event would have added a day, throwing off the prediction, yet later passages confirm that the period from His death to resurrection was in fact three days. 1 Corinthians 15:4 "And that he was buried, and that he rose again the third day according to the scriptures:”
As shown elsewhere in scripture, a day is a set amount of time, rather than a cycle of light and dark. Applying the same to Genesis 1, all six days, with or without the sun should be considered 24-hour days.
The Gap Theory attributes an unspecified amount of time between Genesis 1:1 and 1:2:
(Genesis 1:1) In the beginning God created the heaven and the earth.
(Genesis 1:2) And the earth was without form, and void; [etc. etc. etc.]
In support of the theory, some say that “was” should be translated as “became" implying anything from the fall of Satan to conflict and judgement involving a pre-adamic civilization which plunged the earth into disorder. But this modification to the text would conflict with other passages based on timeline alone. For example, at the giving of the law in Exodus 20, God uses the days of creation as a precedence for the law of the sabbath.
(Exodus 20:9,10a, 11) “Six days shalt thou labour, and do all thy work: But the seventh day is the sabbath of the LORD thy God: . . . For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it."
Exodus 20:11 does not allow for creation of the universe and then a long period of time before creation of the earth. Nor does it allow time for a creation, a conflict, then a recreation because “in six days the LORD made heaven and earth, the sea, and all that in them is.”
One creative application of scripture suggests that Job 38:4-7 supports an old earth because the morning stars sang together and the sons of God shouted for joy (Job 38:7) when God laid the foundations of the earth (Job 38:4). Assuming Job 38:7 is talking about angels, this observation posits that angels were created prior to creation of the earth. Some have used this to say the Bible does support an old earth as a single day for creation of the heavens and the earth would not give enough time for the angels to sing together and shout for joy. Pre-existing angels therefore implies a gap between the creation of the heavens and the earth in Genesis 1:1. However, looking at the Genesis 1 account we can see two uses of the word earth. In Genesis 1:1 and 1:2 earth seems to be used of the planet (Day 1), however in Genesis 1:10, God names the dry land earth (Day 3). 2 Peter 3:5 says that the earth was formed out of water and by water (NIV). This follows the Genesis 1 creation account. The world was made of water and then the land, which God calls earth, comes from the water on Day 3. It does not make sense to lay foundations for a liquid, however forming land from water would require something to foundate the land and keep it from sinking. Job 38:4 is talking about laying the foundations of the dry land on Day 3, not creation of the heavens and the planet earth on Day 1. Creation of the angels with the heavens and the earth on Day 1 or 2 is a plausible conclusion which allows them to sing together and shout for joy as God laid the foundations of the dry land.
Undoubtedly, the theories discussed above come from a genuine effort to harmonize inerrant scripture within the accepted scientific timeline, however details about the Biblical account of creation as well as clarity concerning the Author's intent can be found elsewhere in Scripture.
(Photo Source: http://en.wikipedia.org/wiki/File:Earth_Eastern_Hemisphere.jpg)