When Christians or any religious follower is confronted with evidence that contradicts their belief, they then have in my opinion 3 options. First, deny that the evidence is true and must somehow be wrong. Second, accept the evidence and accept the idea that God gave man a story he could understand at the time to teach lessons of morality even if not completely true. Third, accept the evidence and reject your previous belief that what you were reading was the word of God.
There are other options besides these, such as understanding that scripture is an expression of the human spirit in search of God, and that's its language is "as if" language, that "as if" God were saying this to man; a metaphor in other words, how they modeled reality at the time. Even if in their beliefs at the time they genuinely believed it was 'factually' so, how it functioned was just that, inspiration that 'brought God to earth" for them, so to speak. Understanding this today in light of the study of the humanities in anthropology, mythology, sociology, and so forth allows us to better
contextualize its origins and its nature, thus removing superimposed modern mythologies, such as the Chicago Statement on Biblical Inerrancy, which cannot stand up to critical scrutiny. In this sense, meaning is allowed to be
liberated from the symbol, which the myth of inerrancy amounts to, and to be enjoyed in a fuller, more inclusive context that acknowledges the validity of other faith claims.
There are of course other ways modern Christians grapple with these questions brought to bear for them in the light of modernity, other than burying of the head in the sands of denialism, such as gives birth to fundamentalist strains of the religion.
I find most Christians go with the second option because they believe the bibles true purpose is to inform man on how to live life not on how life was made.
I tend to see most doing things along the lines of still trying to make it 'fit' into the modern myth of inerrancy, which stands as a sort of invisible backdrop in their thinking, not readily examined head on, as it functions somewhat foundationally for them because of how it has been presented by the purveyors of the modern myth. This is whom the OP is going after, trying to prove "a day" is a literal 24 hour day, and thus throwing his support behind Bishop Usher's 17th century dating of the earth (this is how Christians came up with a highly inaccurate 6000 year age of the earth, based on Usher's splicing together the genealogies of the Bible, which is of course a flawed and ridiculous approach in light of the tools of the modern sciences). So the challenge is to say "no a day is not an epoch, but a literal 24 hour period, thus making Usher right." Both of these are flawed, but the latter is far more in denial than the former which is in the process of breaking away from the weak idea of inerrancy as fact.
But beyond this, most mainline traditionalist Christians don't really have a dog in that fight. They just find the stories to be meaningful and don't really examine it at that frankly modern level. If asked, "Did Jesus really walk on water," they might respond, well, maybe, but that's not the meaning of it to me." This whole business of the OP seems to be getting back to the "fundamentals" but it is in fact not at all! It's thoroughly modernistic in nature, just horribly inadequate at that level.
Here's a great quote to share from an essay by Conrad Hyers who is a professor of comparative mythology at a University near where I live.
"One of the ironies of biblical literalism is that it shares so largely in the reductionist and literalist spirit of the age. It is not nearly as conservative as it supposes. It is modernistic, and it sells its symbolic birthright for a mess of tangible pottage. Biblical materials and affirmations -- in this case the symbolism of Creator and creation – are treated as though of the same order and the same literary genre as scientific and historical writing. “I believe in God the Father Almighty” becomes a chronological issue, and “Maker of heaven and earth” a technological problem."
I don't think I could say it better than this. This is not conservative truth, it is reactionary, trying to use the tools of modernity to hang on to the myths of the past as if they are the sole and only source of finding truth and meaning about God. It is the opposite of faith, IMO. It runs and hides from faith being purified in the fires of sensible knowledge.
The formation of the universe and evolution of life are very complex subjects and would be impossible even for god to describe to people 2,000 or more years ago.
Or, not be in the minds of the authors of Genesis at the time they shared their perspectives of life and reality, touching on the existential questions of being and the struggle of life, the nature of suffering, and so forth. The stories are simply vehicles that carry their thoughts about these things. It's NOT about teaching science and history!