When The Fundamentals (from whence we get the derogatory term "fundamentalist") were written at the beginning of the 20th century, they argued for an ancient earth and universe and in particular for the day-age interpretation -- that the days of creation should be understood as long periods of time. However, they didn't merely appeal to the Bible, but to Church history as well: "Do not think that this larger reading of the days is a new speculation. You find Augustine in early times declaring that it is hard or altogether impossible to say of what fashion these days are, and Thomas Aquinas, in the middle ages, leaves the matter an open question" (see also here). Astronomer Hugh Ross has documented in Creation and Time and A Matter of Days plenty of Christians throughout Church history who argued that the days of creation were long periods of time. Christian philosopher Kenneth Richard Samples, a colleague of Ross, writes, "From the time of the Church fathers, through the Reformation, and up to the present, various views have prevailed, some more broadly represented than others, but none was ever considered the definitive, or the only, orthodox biblical position." Similarly, the 19th century theologian William G. T. Shedd wrote in his Dogmatic Theology:
The very common assertion, that the church has altered its exegesis, under the compulsion of modern geology, is one of the errors of ignorance. The doctrine of an immense time, prior to the six creative days, was a common view among the fathers and schoolmen. ... Respecting the length of the six creative days, speaking generally, for there was some difference of views, the patristic and mediaeval exegesis makes them to be long periods, not days of twenty-four hours. The latter interpretation has prevailed only in the modern church.
In response to this, young-earth proponents state that these claims are simply false: for example, in The Genesis Debate which has proponents of three different interpretations of Genesis 1 defend their positions, J. Ligon Duncan III and David W. Hall write "This claim [that there was no exegetical consensus on the days of Genesis] is wrong. There was only one view following the repudiation of Augustine's view, and seldom (if ever) before the nineteenth century was the day-age or the framework view advocated." Similarly, in another book Mark Van Bebber and Paul Taylor write, "One is caused to question whether Dr. Ross has actually read any of the writings he quotes. Most of the 'Church Fathers' he claims as believing in figurative 'days' actually believed just the opposite. This is even evident within the same context of the quotes Ross reported. Creation and Time misinterprets no fewer than 9 of the 14 men listed." Unfortunately, the title of the latter book is the same as the book they are criticizing: Creation and Time. It's difficult to accept their condemnation of Ross's alleged misrepresentations when they gave their book the same title as his, and had Ross's name (in the subtitle) in larger font on the cover than their own -- all without permission (Ross, Matter of Days, 262, n. 7). I could certainly be wrong, but it looks like they were trying to trick people into thinking they were buying his book instead of theirs. Moreover, they don't give any reason for thinking the day-age interpretation is figurative rather than literal; and I don't understand why they found it necessary to put "Church Fathers" in scare quotes.
As far as I can understand them, there are plenty of statements in the Church fathers which, at the very least, strongly imply that some of them understood the days of creation as calendar days. Conversely, in defense of the view expressed by Shedd, Ross, Samples, and The Fundamentals, it must be recognized that there are also plenty of statements in the Church fathers which claim or imply that the days of creation were millennia. For example, some of the fathers argued that God's statement to Adam that he would die "in the day" that he ate the forbidden fruit (Gen. 2:17) referred to the sixth day of creation. Since the time between Adam's creation and his death was slightly less than a thousand years (Gen. 5:5), and since a thousand years is as a day to the Lord (Ps. 90:4), the sixth day (when Adam was created) was a thousand years long (see, i.e. Justin Martyr, Dialogue With Trypho, 81; Irenaeus, Against Heresies, 5:23:2). Others, in similar language as Hebrews 4, identified the present or (more often) the future age with the seventh day of creation, sometimes claiming that it was instituted with Christ's first advent (Irenaeus, Against Heresies 30:4; 33:2; Origen, Against Celsus, 6:61).
There's more to the story, though. Prior to the Reformation, most biblical commentators believed in multiple interpretations of Scripture. That is, any given passage had several different meanings. And it seems apparent that the Church fathers understood the day-age interpretation as a secondary one; that is, the days of creation referred to long spans of time in addition to their primary meaning.
This is evident from the fact that they thought each day was an age of human history. For example, they often understood the sixth day of creation as referring to the 1,000 years prior to Christ. The problem comes in when we remember that human beings weren't created until the sixth day. So if the sixth day referred to both the thousand-year period leading up to Jesus' time and the time when human beings were first created, were they suggesting that Adam and Eve were created sometime within the millennium before Christ? Were they suggesting that all of human history -- from Noah to Abraham to Moses to David to Nehemiah, etc. -- could be squeezed into the 1,000 years leading up to Jesus' birth? Of course not. Obviously, understanding the sixth day as a millennium is a secondary interpretation.
So what can we conclude from this? Not as much as some might hope. For one thing, the recognition that it was a secondary interpretation does not negate the fact that it was still a common interpretation. Regardless of whether it was secondary, the early Church fathers still understood the days of creation to be reasonably interpreted as long periods of time. They gave exegetical arguments for it. Perhaps those arguments were not sound, but it can't be claimed that the day-age interpretation is just a recent position. Part of the problem here is that the ancient and medieval exegetes thought that the various interpretations of Scripture fell into particular categories, and so they read these categories into the Bible, even where they didn't fit. This often led to interpretations which were highly subjective and forced. But this is not the case for the days of creation. They gave exegetical arguments for why they should be understood as long periods of time, and these arguments were widely accepted. Moreover, the point is not whether we should understand the Bible as having several different interpretations (Catholics do, Protestants don't, I make no comment here) but whether the early Church fathers did.
For another thing, young-earth proponents seem to assume that since the day-age view was a secondary interpretation, the primary interpretation was the calendar-day view. But this doesn't seem clear. There are few explicit statements to this effect, and we know that some of the fathers, most notably Augustine, thought that all of the events of creation week took place instantaneously. Others argued that some aspects of the syntax in Genesis 1 were unusual, and shouldn't be understood superficially. Origen went so far as to mock those who took the creation stories in Genesis 1-3 as historical (De Principiis, 4:1:16) The point being that there was controversy in the early Church over how the creation narrative should be interpreted, and this controversy extended to the nature of the creation days. If a Christian writer of the time thought that the days of creation were calendar days, he would have had to make that clear to his readers since some of the most prolific Christian authors denied it. In the absence of such clarifications, there is simply no reason to assume that the calendar-day interpretation was universally presumed as the primary interpretation. How do we know that the "instantaneous" view wasn't the primary interpretation?
Thus far, I have been discussing what the Church fathers wrote about the days of creation, rather than what they wrote about the age of the universe. But their understanding of the latter can be easily determined by reflection upon the former: if they thought that the six creation days were each 1,000 year periods, or that all of the events in Genesis 1 took place instantaneously in no time, how old would they have concluded the universe is? Well, several thousand years. They state this pretty explicitly. Because of this, many young-earth proponents think that their views are much closer to those of the early Church, and with some justification.
However, it must be borne in mind that the reason contemporary young-earth proponents make this claim is because they insist that the days of creation can only be validly interpreted as referring to calendar days. In other words, they have different reasons for believing the earth to be several thousand years old than Christians have historically.
The significance of this is twofold: first, an argument is only as good as its premises. Since modern young-earth proponents disagree with the traditional premises that the creation days are millennia (the day-age view) or that they're metaphorical (the instantaneous creation view), their "agreement" that the universe is several thousand years old becomes irrelevant.
Second, modern day-age proponents are closer to the beliefs of the early Christians, since they agree with one of their premises, that the days of creation refer to long periods of time. They disagree that they were long periods of a specific length (1,000 years), and that this constitutes a secondary, rather than the primary, interpretation. But their agreement is more significant than the superficial one between modern young-earth proponents and the Church fathers.
I've been emphasizing the views of the early Church since this is the focus, or one of the foci, of both sides of the debate. I haven't studied the medieval exegetes enough to say much about them, other than there doesn't seem to be much alteration of the themes discussed above. Augustine's view of instantaneous creation was commonly accepted. Anselm, for example, wrote (in Cur Deus Homo, 1:18)that "the whole creation took place at once, and those days in which Moses appears to describe a successive creation are not to be understood like such days as ours."
With the Reformation, Protestants rejected multiple interpretations of the Bible and, while respecting the views of Christians throughout history, did not let traditional interpretations determine their understanding of the biblical text. Many accepted the calendar-day interpretation, including Martin Luther, but he also wrote in The Creation that the creation account "contains things the most important, and at the same time the most obscure," and, in light of all the differing interpretations of it made before his time, despaired of ever truly understanding Genesis 1 beyond the simple facts "that the world began, and was made of God, out of nothing."
Protestants formed creeds to summarize their positions regarding the most important aspects of Christianity. Very few of them addressed the nature of the creation days or the age of the universe. The Belgic Confession merely states that God created everything "when it seemed good to him" (quand bon lui a semblé; art. 12), which suggests its authors were being deliberately agnostic about the universe's age.
The Protestant confession that is most often appealed to by young-earth advocates is the Westminster Confession. Some of the 151 authors expressed elsewhere a belief in the calendar-day interpretation (five or fifteen of them depending on who you ask), and the Confession itself states that God created everything "in the space of six days" (ch. 4). From this it is claimed that the Westminster Confession affirms the calendar-day interpretation.
I think this is reading too much into the Confession. I agree, after all, that God created everything in the space of six days. I disagree, however, that he created everything in the space of six calendar days. The Westminster Confession does not say that the days of creation should be understood as 24-hour periods; and, again, given the diversity of views on this issue, if they were intending to do so, they would have had to make this much more explicit. The faculty of Westminster Theological Seminary, which subscribes to the Westminster Confession, commissioned a report on this issue, and concluded that the phrase "in the space of six days" was not meant to define the days of creation as calendar days. Rather, it was intended to counter (but not condemn) the Augustinian view that creation took place instantaneously and took no time.
... we recognize that the exegetical question of the length of the days of Genesis 1 may be an issue which cannot be, and therefore is not intended by God to be, answered in dogmatic terms. To insist that it must comes dangerously close to demanding from God revelation which he has not been pleased to bestow upon us, and responding to a threat to the biblical world view with weapons that are not crafted from the words which have proceeded out of the mouth of God.
So there were a multiplicity of views on how to understand the creation narrative in Genesis 1 throughout Christian history, long before the discoveries of modern science that the universe is ancient. The day-age interpretation in particular is neither new nor unusual. If it is objected that no one predicted that the universe is billions of years old from the Bible alone, I simply respond that the whole point of these posts is that the Bible does not give us enough information by itself to determine a specific age of the universe, so I don't consider the fact that no one did to be a failing.
The study commissioned by Westminster Theological Seminary makes a further point, that this issue "never seems to have been regarded as a test of orthodoxy in the reformed churches." Perhaps the most important thing to take away from all of this is that historically Christians have not considered the length of the days of creation a significant issue, and they were tolerant of different interpretations. This point is made best by Gleason Archer and Hugh Ross in their contributions to The Genesis Debate:
Prior to 1650 exegetes gave little attention to the length of the creation days. Of the approximately two thousand extant pages of creation-day commentary by early Church fathers, only a total of about two pages address the duration of the creation days. Anyone who reads the original source literature will notice the difference in tone between the early Church fathers and modern 24-hour advocates. The older writings are devoid of passionate certainty and dogmatism about the length of the creation days. Rather, they evidence a tentativeness and exhibit tolerance on this point.
Since this topic often generates more heat than light among Christians, and since it is an excuse that certain secular forces in society use to justify rejecting Christianity, I suggest we should try to imitate the humility of those who have gone before us in the way of Christ.
(see also part 1 and part 2)