When it comes to the language of the Westminster Standards, it is safe to say that the Westminster Divines, those theologians, pastors, and others assembled to craft the theological statements they published, were standing on the shoulders of theological giants.
One such Reformed theologian was John Calvin. When writing the answer to WSC 9, they borrowed language directly from Calvin when it came to the time span of creation. Compare:
“Let us rather conclude that God himself took the space of six days, for the purpose of accommodating his works to the capacity of men.” – John Calvin – Genesis Commentary
With WSC9:
The work of creation is, God’s making all things of nothing, by the word of his power, in the space of six days, and all very good.
With WLC 15:
Answer: The work of creation is that wherein God did in the beginning, by the word of his power, make of nothing the world, and all things therein, for himself, within the space of six days, and all very good.
The Westminster Confession of Faith, Chapter 9 “Of Creation”
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.
Can we call John Calvin a Westminster Divine? Well, not technically, but he would certain qualify for honorary status.