Genesis 1:5

And the evening, etc.] RV 'and there was evening and there was morning, one day.' In the endeavour to bring the Creation story into harmony with the ascertained results of science, it is often maintained that the writer meant indefinite periods of time by the term 'days.' But the science of Geology was entirely unknown to the ancients, and it is not legitimate to read a knowledge of modern discoveries into these ancient records. The author meant days in the sense of Genesis 1:16. Evidently, he had in mind the Jewish week, which he regarded not only as prefigured, but rendered obligatory, by God's example in creating the world, as God worked six days, and rested the seventh: so the week was to consist of six working days, and a Sabbath day of rest. At the same time the writer intended to show that there was an orderly process in the work of creation. Note that evening is put before morning, probably because the Jewish day began at sunset.

6-8. Second day:—Creation of the Firmament.