Evening and morning

Genesis 1 has the refrain: And there was evening and there was morning, the nth day.

Much has been written about the meaning of the word day (Hebrew yom). Clearly in Genesis 1:5 it means daylight.

God called the light Day, and the darkness he called Night. (ESV)

In Genesis 2:3 to it refers to period of time

...in the day that the LORD God made the earth and the heavens. (ESV)

leading to translations like "when"

When the LORD God made the earth and the heavens (NIV)

The word "day" joined to a number implies the sense of a 24-hour day.

And there was evening and there was morning, the third day. (Gen 1:13 ESV)

Young earth creationists often use the qualifier "24-hour" so that their meaning is clear: "The world was made in 6 24-hour days." It is hard to misunderstand that. So me may complain that Genesis should be that clear if it intends to say this. I think it is and that it is extra-biblical ideas that challenge the straightforward interpretation. My question back is: "If God did indeed create in 6 days, how else could he have written Genesis 1?" Whereas if it really was over long periods of time there are plenty of other ways to show this.

But I think Genesis does say something along the lines of what young earth creationists say: that the world was made in 6 24-hour days; just it is written in a different way than we expect. The issue is that hours are a human construct. A reasonable one but there is no phenomenon that suggests the rotation of the earth should be divided into 24 periods. So the creation record we have in Genesis could not have been written "24-hour day." because the concept of hours was not yet in existence (the word "hour" appears twice in the Old Testament). So if God was to say the equivalent of "24-hour" to modify day he could use a division of the day to say this. At the creation there is a natural division: daytime and night-time. So the equivalent to "24-hour day" at the time of creation is a "2-time-period day," which is what we read:

And there was evening and there was morning, one day. (Gen 1:5)

And there was evening and there was morning, a second day. (Gen 1:8)

And there was evening and there was morning, a third day. (Gen 1:13)

And there was evening and there was morning, a fourth day. (Gen 1:19)

And there was evening and there was morning, a fifth day. (Gen 1:23)

And there was evening and there was morning, the sixth day. (Gen 1:31)