In the Space of Six Days
The Westminster Confession makes over 4000 direct references to and quotations of the Scriptures. "In the space of six days" is not a reference to or quotation of the Scriptures.
The Westminster Confession of Faith 4.1 says: "It pleased God the Father, Son, and Holy Ghost, for the manifestation of the glory of His eternal power, wisdom, and goodness, in the beginning, to create, or make of nothing, the world, and all things therein whether visible or invisible, in the space of six days; and all very good."
"In the space of six days" is not a biblical phrase, but a theological phrase, intended to interpret the Scriptures rather than quote them.
Basil, referencing Origen said, “This beginning was instantaneous... in a rapid and imperceptible moment." Origen said the world as we know it was created in one day, but many of the ancients thought the world was eternal (including Origen). They reflect Plato who said that the world was created out of preexistent matter.
Augustine also believed in an instantaneous creation. He wrote, “[God] created all things simultaneously and also created this one day, seven times repeated. The need for these six days to be set out was to teach those who could not understand simultaneous creation…God accommodated himself to the capacity of weaker intellects and presented creation as if it were a process." (Works, 4.51-52; 5.5)
Medieval thinkers followed suit with Augustine. Bede, Anselm, and even Aquinas followed Augustine. But Aquinas "distinguished" in scholastic format saying that the creation was three-fold and there was an adornment process that occurred. He wrote, “On day four lights are given to adorn the heaven. On day five birds and fish are created to make beautiful the intermediate element. On day six animals are brought forth to live and move on the earth.” The creation narrative was intended to help us to see the creation in frames, adornment being one of the three.
Luther would come along question the medieval interpretations of "day" in creation. He said, in Lutheresque fashion, “There had not been anyone in the history of the church who had explained everything in the chapter with adequate skill.” He saw six days as six days. In his Works, 1.3-5, he writes, "that the world, with all its creatures, was created within six days, as the words read."
So does our Westminster Confession employ the language of Luther? No, it is the theological language of Calvin that the Westminster divines employ.
Calvin said, “With the same view Moses relates that the work of creation was accomplished not in one moment, but in six days. By this statement we are drawn away from fiction to the one God who thus divided his work into six days, that we may have no reluctance to devote our whole lives to the contemplation of it. For though our eyes, in what direction soever they turn, are forced to behold the works of God, we see how fleeting our attention is, and holy quickly pious thoughts, if any arise, vanish away. Here, too, objection is taken to these progressive steps as inconsistent with the power of God, until human reason is subdued to the obedience of faith, and learns to welcome the calm quiescence to which the sanctification of the seventh day invited us.” Institutes, 1.14. In his Commentary on Genesis he would elaborate on this in those words now immortalized in the Westminster Confession: "God himself took the space of six days” to create the world.
So the Westminster Confession with its over 4000 direct quotes of the Scripture, chose not to leave creation to a direct quote in 4.1, but instead to interpret the creation. Not as the ancients in seeing creation as instantaneous; not as the medievals who saw adornments and literary features; but as Calvin would understand it--a literal six day creation. "In the space of six days" is a theological language that comes to the Westminster Confession through Calvin. I think it is clear how the divines intended it to be understood.