Secondly, the literal reading, which merely states that the meaning of the words are 'normal" unless a modifier is included, does not support a 'day' longer then a evening-morning cycle.
You are importing your conclusion into your premises by use of that sneaky word "normal".
Let me explain with an example. Suppose I were to argue that Mary was not a virgin when she gave birth to Jesus. My evidence is: "... Jacob the father of Joseph the husband of Mary, of whom Jesus was born, who is called Christ." (Matt 1:16, ESV) Now, everywhere else in the Bible, when a child is born to a woman, that woman has had sex with someone. That is the "normal" meaning or implication of "born". So, as a normal mother, Mary must have also had sex with Joseph in order to be conceived with Jesus!
Can you see how the argument fails? The conclusion I wanted to reach was actually "Mary was a normal mother". But I
assumed it in the course of the argument. And in this case the assumption (heh!) is invalid, but the "modifier" that you speak of is not so much in the text (i.e. in verse 16 itself) as it is in the
context: if we read on a little further, we find that Mary was in fact as abnormal a mother as you could get!
So it is with Genesis 1. If you simply insist that the days of Genesis 1 are "normal" days, you are really assuming what you actually set out to prove. And again, there is no evidence in the text itself ("there was evening, and morning, the first day") to indicate that anything other than a normal day is meant - but the
context leaves room for the days to be abnormal. The earth is formless and void. The Spirit is hovering over the waters. God is poofing things into existence left, right, and center. There isn't even a Solar System in existence for the first three days! Those obviously aren't normal conditions, any more than having an angel announce your baby's name is a normal conception.
The most common canard at this point is that the Earth could have been rotating about its own axis and that rotation would have defined the length of the day. The first thing to note is that there is absolutely nothing in the text of Genesis 1 (or indeed in the rest of Scripture, as far as I'm aware) to indicate that the Earth is at any point rotating about its own axis. The creationist has imported secular, extra-Biblical science! The second thing to note is that even were the Earth rotating, are we told that its period of rotation in Genesis 1 was 24 hours long? Hardly. Again the creationist is importing extra-Biblical science (since the Bible itself never specifies the length of a day, assuming that its readers will measure it without supernatural help).
But most importantly, this does not follow a literal interpretation of the text, the very thing creationists claim to be upholding. For what does the text say determines the length of the day?
And God said, “Let there be lights in the expanse of the heavens to separate the day from the night. And let them be for signs and for seasons, and for days and years ... " (Gen 1:14, ESV)
In other words, if one wishes to be literalistic, it is the
heavenly lights that determine the length of the day, not any intrinsic movement of the Earth! Now there are three days before Gen 1:14; what determined their length? Maybe God did, or maybe some unknown astronomical reference point did, but
it was not the Earth's rotation itself which determined the length of the day. For if it was so, why couldn't the Earth's rotation also determine the length of the day in Gen 1:14? If God meant to say "And let the motions of the Earth under the lights be for signs and for seasons, and for days and for years", why didn't He just ...
say so?
So, ironically, the text itself gives ample proof that the first three days need not have been exactly the same length that days are today.