Day-Age Creationism is a type of Creationism that holds that the six days referred to in the Genesis account of creation are not ordinary 24-hour days, but rather are much longer periods (of thousands or millions of years). The Genesis account is then interpreted as an account of the process of cosmic evolution, providing a broad base on which any number of theories and interpretations are built.
This group notes that God is not bound by time and that the term "day" in Genesis is used before the creation of the sun or the moon; consequently, "day" does not refer to an Earthly day, because such a day does not yet exist. This abstract use of "day" is found in other mythological and religious writings of the Middle East to denote the passage of cosmic benchmarks in addition to referring to earthly time marked by the sun or the moon.
Day-age creationism suggests that the very brief account of creation in Genesis was not intended as informative, but rather as a succinct summary of ancient knowledge in the Levant. Moreover, it implies that the Creation narrative is brief because it serves as an introduction linking the rise of the Judaic ethnos to the dawn of time. Thus, to ascribe any specific and definitive interpretation is beyond the scope and intent of the passages in Genesis and is by nature subjective and controversial. Moreover, to require that faith in God be contingent on any one interpretation of creation is to limit a believer's faith to earthly and not spiritual matters.
However, there are some major problems with day-age creationism, such as that God would have created plants an age before the sun, an impossiblity. Which is sometimes countered with the explanation that the "light" from the first "day" was gradually introduced, as may be indicated by translator J. W. Watts: “And gradually light came into existence.” Another explanation is that the Sun was created on the first day, but the translucent layer of slight darkness (1:3) was not removed until the fourth day, rendering the Sun invisible but still with visible light during the first three days. Some scholars claim that according to contextual, linguistic and grammatical evidence, the most probably interpretation of "day" in this context is a "period of time of length equal to what we now call a day", and on this basis they reject Day-Age creationism.
That said, Day-Age Creationism does include a number of specific and interesting theories and speculation regarding divine creation of the universe which is ultimately unknown to man. These ideas revolve around the use of the word "day" in contexts before the existence of the sun, moon, and earthly time in the book of Genesis.
No language is very rich in words significant of definite periods of time; but in the early Hebrew they seem to have been very scanty. The day, week, month, year, and generation (this last usually implying the time from the birth of a man to that of his son, but possibly in Gen. 15:16, a century) are all that we find. These in their literal sense were evidently inadequate. There was no word at all in early Hebrew equivalent to our words "period" and "season." When such an idea was to be expressed, it was done by the use of the word "day," either in the singular, or more commonly in the plural.
Thus, "the time of harvest;" "the season of the first ripe fruit," are literally "the days of harvest," "the days of the first ripe fruit." In Isaiah 34:8, the singular is used, and followed by the word year in the same indefinite sense. "It is the day of the Lord's vengeance, and the year of recompenses for the controversy of Zion." The broad outline found in Genesis 1:1 through 2:3 ends in 2:4 with the words: "in the day that the LORD God made the earth and the heavens":  (http://www.blueletterbible.org/tmp_dir/c/1095203603-5111.html). The word "day" that's used here in reference to the period of all the creative days together is the same Hebrew word (yowm) used for each of the six creative days in chapter one.
In the 14th and following verses, when the author is describing the formation of the heavenly luminaries, he is particular in mentioning that one part of their office was to "rule over the day and over the night, and to divide the light from the darkness." Hence it is sometimes inferred that he was mistaken in speaking of day and night at an earlier period. But such a mistake seems incredible. If then, under such circumstances, he uses the word "day" long before he comes to the formation of the sun, the natural inference is that he did so designedly — that it was his intention that his readers should understand that he was speaking of something very different from that natural day which is regulated by sunrise and sunset. To explain this, some creationists who hold the view that the six day period in the Genesis account of creation refers to the time spent by light traveling from the center of the universe at the time and point of creation.
- A web site advocating the Day-Age Theory (http://www.godandscience.org/apologetics/day-age.html)