The following are quotes of church fathers who may have been open to the day-age theory:
"For as Adam was told that in the day he ate of the tree he would die, we know that he did not complete a thousand years [Gen. 5:5]. We have perceived, moreover, that the expression ‘The day of the Lord is a thousand years’ [Ps. 90:4] is connected with this subject" (Dialogue with Trypho the Jew 81 [A.D. 155]).
"And there are some, again, who relegate the death of Adam to the thousandth year; for since ‘a day of the Lord is a thousand years,’ he did not overstep the thousand years, but died within them, thus bearing out the sentence of his sin" (Against Heresies 5:23:2 [A.D. 189]).
Clement of Alexandria
"And how could creation take place in time, seeing time was born along with things which exist? . . . That, then, we may be taught that the world was originated and not suppose that God made it in time, prophecy adds: ‘This is the book of the generation, also of the things in them, when they were created in the day that God made heaven and earth’ [Gen. 2:4]. For the expression ‘when they were created’ intimates an indefinite and dateless production. But the expression ‘in the day that God made them,’ that is, in and by which God made ‘all things,’ and ‘without which not even one thing was made,’ points out the activity exerted by the Son" (Miscellanies 6:16 [A.D. 208]).
"The first seven days in the divine arrangement contain seven thousand years" (Treatises 11:11 [A.D. 250]).
"For who that has understanding will suppose that the first and second and third day existed without a sun and moon and stars and that the first day was, as it were, also without a sky? . . . I do not suppose that anyone doubts that these things figuratively indicate certain mysteries, the history having taken place in appearance and not literally" (The Fundamental Doctrines 4:1:16 [A.D. 225]).
"The text said that ‘there was evening and there was morning’; it did not say ‘the first day,’ but said ‘one day.’ It is because there was not yet time before the world existed. But time begins to exist with the following days" (Homilies on Genesis [A.D. 234]).
"And since he [the pagan Celsus] makes the statements about the ‘days of creation’ ground of accusation—as if he understood them clearly and correctly, some of which elapsed before the creation of light and heaven, the sun and moon and stars, and some of them after the creation of these we shall only make this observation, that Moses must have forgotten that he had said a little before ‘that in six days the creation of the world had been finished’ and that in consequence of this act of forgetfulness he subjoins to these words the following: ‘This is the book of the creation of man in the day when God made the heaven and the earth [Gen. 2:4]’" (Against Celsus 6:51 [A.D. 248]).
"And with regard to the creation of the light upon the first day . . . and of the [great] lights and stars upon the fourth . . . we have treated to the best of our ability in our notes upon Genesis, as well as in the foregoing pages, when we found fault with those who, taking the words in their apparent signification, said that the time of six days was occupied in the creation of the world" (ibid., 6:60).
"For he [the pagan Celsus] knows nothing of the day of the Sabbath and rest of God, which follows the completion of the world’s creation, and which lasts during the duration of the world, and in which all those will keep the festival with God who have done all their work in their six days" (ibid., 6:61).
"Therefore let the philosophers, who enumerate thousands of ages from the beginning of the world, know that the six-thousandth year is not yet complete. . . . Therefore, since all the works of God were completed in six days, the world must continue in its present state through six ages, that is, six thousand years. For the great day of God is limited by a circle of a thousand years, as the prophet shows, who says, ‘In thy sight, O Lord, a thousand years are as one day [Ps. 90:4]’" (Divine Institutes 7:14 [A.D. 307]).
"Seven days by our reckoning, after the model of the days of creation, make up a week. By the passage of such weeks time rolls on, and in these weeks one day is constituted by the course of the sun from its rising to its setting; but we must bear in mind that these days indeed recall the days of creation, but without in any way being really similar to them" (ibid., 4:27).
"[A]t least we know that it [the Genesis creation day] is different from the ordinary day with which we are familiar" (ibid., 5:2).
"For in these days [of creation] the morning and evening are counted until, on the sixth day, all things which God then made were finished, and on the seventh the rest of God was mysteriously and sublimely signalized. What kind of days these were is extremely difficult or perhaps impossible for us to conceive, and how much more to say!" (The City of God 11:6 [A.D. 419]).
"We see that our ordinary days have no evening but by the setting [of the sun] and no morning but by the rising of the sun, but the first three days of all were passed without sun, since it is reported to have been made on the fourth day. And first of all, indeed, light was made by the word of God, and God, we read, separated it from the darkness and called the light ‘day’ and the darkness ‘night’; but what kind of light that was, and by what periodic movement it made evening and morning, is beyond the reach of our senses; neither can we understand how it was and yet must unhesitatingly believe it" (ibid., 11:7). http://www.catholic.com/Library/Cre...and_Genesis.asp
Though it is highly paradoxical that the Hexaemeron could represent six indefinite periods of time while the story of Adam and Eve is a historical account, this may nonetheless be true.
The notion that each day should be understood "befittingly of God" is just as rational as believing that He created the universe with "apparent age".
Given that the Scripture is not explicit on this matter, it is perhaps best to leave it as a mystery and focus on what we can be more certain of.
Given that God stands outside of time, He did not necessarily create within time and therefore, the universe itself could be billions of years old even though the Creation ocurred in six "days". Given that the fathers of the Church were not unanimous in the meaning of the Hexaemeron, one need not believe in a young universe in order to uphold the authority of Scripture.
God created the earth in six days and specially created the species and mankind. Genesis itself is a factual history. The nature of time before the fall, on the other hand, is a mystery to human understanding. The universe may be billions of years old or less than ten thousand. However, if the universe is young, then the supernovas we observe never really happened. That is a little unsettling.
As sort of a "patristic scholar", I am constantly learning more and more about what the fathers of the Church taught. Given that the fathers were not unanimous of the length of the Creation days, we should be open to the possibility that modern science is correct in its dating techniques.
As I have shown before, some of the fathers taught that we must understand the days of the Hexaemeron in a manner befitting of God, given that He stands outside of time.
May peace be upon thee and with thy spirit.