The book of Genesis seems to divide the work of creation into six successive steps of stages. It is certainly not necessary to think that the six days spoken of in the first chapter of the Bible are intended to be six days of twenty-four hours each. We may think of them rather as very long periods of time. But do they not at least mark six distinct acts or stages of creation, rather than merely six periods in which God molded by works of providence an already-created world? [J. Gresham Machen, Things Unseen: A Systematic Introduction to the Christian Faith and Reformed Theology (Glenside, PA: Westminster Seminary Press, 2020), p. 228)
On the other hand, to the person who does not share those naturalistic presuppositions, that leap from the actual evidence to the evolutionary hypothesis will seem to be a reckless leap indeed. To the person who does not believe that Jesus Christ was a product of evolution, but who believes that he came into this world by a stupendous miracle, the testimony to an equally supernatural origin of the first man will seem to be overwhelming. Such a person will say with great confidence not that man is a product of evolution but that God created man. (Ibid.,p. 234)
As it can be seen, Machen is agnostic over the issue of 6-24 creation, but appears to hold the Day-Age view of creation days. On the issue of creation versus evolution however, Machen emphatically rejects any form of evolution with regards to humans. Machen would thus have rejected Theistic Evolution if it was present at his time.