You said this
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For young earthers, day one necessarily begins in either verse 1 or verse 2.
If verse 2, that leaves verse 1 as a general statement that God created everything, and all that follows provides the details of what occurred in verse 1. So, verse 2 is not necessarily a continuation of verse 1, but a detailed reiteration and explanation. (Just like how the general statement of the creation of man and woman in Gen. 1:27 is detailed in chapter 2.) However, that ignores the fact that days two through six begin with "And God said," suggesting that day one begins in verse 3.
My point is that the earth, along with the rest of the universe, could have been created in verse 1, but was only created to the point of being "without form and void;" merely water and rock (verse 2). Further specific creation to allow for human and animal habitation was then done in days one to six. Hence why it is possible that day one begins in verse 3"
The first day when it was created is where it began. So why would we look to some verse after that somewhere and assume that could really the the beginning? We know the heaven and earth was created and then He did some various other things like create life on it. The total is six days. You can't hack of verse one and separate it.
If you read everything I wrote, you would see that I didn't make those statements in a vacuum. It is no more an assumption that day one begins in verse 3 than it is that it begins in verse 1. Again, one issue is that each of the days from two to six begins with "And God said," but that doesn't happen
until verse 3. That could be understood to mean that day one doesn't start until verse 3, giving an indefinite period of time between verses 1 and 2, and or 2 and 3.
Another issue is the lack of definite articles for days one to five. That could suggest that the days aren't consecutive 24-hr days; they are merely a chronological sequence of days which could be separated by many thousands of years.
We do also have "day,"
yom, being used elsewhere in Scripture with several different meanings, including an indefinite period of time.
There is also "day" and "night," "morning" and "evening,"
prior to day four, when God "God made the two great lights—the greater light to rule the day and the lesser light to rule the night—and the stars" (v. 16, ESV).
All I am saying is that there are textual issues, often missed or glossed over by young earthers, that open the door for various, legitimate understandings of the text of Genesis 1 to support an old earth. The Bible does not tell us how old the earth is and so to assume a young earth is the only correct understanding is wrong.