- Day–Age: After each literal 24-hour days of Creation (all six of them), Yahweh permitted a great time to pass — which comprise the geologic ages of Earth — in order to allow His creations to mature & develop gradually.
- Gap: Genesis 1:1 describes an original creative event which subsequently fell into ruin as a result of some kind of disaster, often believed to be Satan’s rebellion. An unknown amount of time — a gap — exists between Genesis 1:1 & 1:2, and the six days of Creation are in effect a recreation.
- Indefinite Age: The six days of Creation are not literal days but are of indeterminate length, allowing for a resolution of the theory of evolution, geologic ages, and the Genesis 1 record. The order of the days is not important.
- Seven Day: The six days of Creation are six true days within which Yahweh created the heavens, the earth, and all that in them is. This Creation took place an indeterminate amount of time ago.
- Revelatory Day: The seven days of Genesis 1 are not seven days in which Yahweh was creating & resting; rather, the seven days are the week in which Yahweh revealed to Moses the events of Creation, which transpired over the course of an unknown length of time in the unknowable past.
- Revelatory Device: The seven days are nothing more than a literary device used by the author of Genesis in order to better organize & present his thoughts.
Source: Baffling Bible Questions Answered by Larry Richards, page 18.
I personally believe that only one of those theories — Seven Day — plays nicely with the whole of the Scriptures, particularly Exodus 20:8–11. However, as Larry Richards says on page 19:
Speculation and argument are always of interest. But this is one question that will not, and need not, be resolved. For the issue is not how God went about creation but that God did create. It is not necessary that we know how he did this. It is necessary that we affirm the fact that he did it. Everything in the material and spiritual world exists because God called it into being. This is the unshakable bedrock of Scripture’s vision of reality.