'Thus evening was and morning was one day.' echad (one), like eis and unus, is used at the commencement of a numerical series for the ordinal primus (cf. Gen. 2:11; 4:19; 8:5, 15). Like the numbers of the days which follow, it is without the article, to show that the different days arose from the constant recurrence of evening and morning. It is not till the sixth and last day that the article is employed (v. 31), to indicate the termination of the work of creation upon that day. It is to be observed, that the days of creation are bounded by the coming of evening and morning. The first day did not consist of the primeval darkness and the origination of light, but was formed after the creation of the light by the first interchange of evening and morning. The first evening was not the gloom, which possibly preceded the full burst of light as it came forth from the primary darkness, and intervened between the darkness and full, broad daylight. It was not till after the light had been created, and the separation of the light from the darkness had taken place, that evening came, and after the evening the morning; and this coming of evening (lit., the obscure) and morning (the breaking) formed one, or the first day. It follows from this, that the days of creation are not reckoned from evening to evening, but from morning to morning.The first day does not fully terminate till the light returns after the darkness of night; it is not till the break of the new morning that the first interchange of light and darkness is completed
, and a [greek] has passed. The rendering, 'out of evening and morning there came one day,' is at variance with grammar, as well as with the actual fact. With grammar, because such a thought would require [hebrew] and with fact, because the time from evening to morning does not constitute a day, but the close of a day. The first day commenced at the moment when God caused the light to break forth from the darkness; but this light did not become a day, until the evening had come, and the darkness which set in with the evening had given place the next morning to the break of day. Again, neither the words [hebrew], nor the expression [hebrew] evening-morning (= day), in Dan. 8:14, corresponds to the Greek [greek], for morning is not equivalent to day, nor evening to night. The reckoning of days from evening to evening in the Mosaic law (Lev. 23:32), and by many ancient tribes (the pre-Mohammedan Arabs, the Athenians, Gauls, and Germans), arose not from the days of creation, but from the custom of regulating seasons by the changes of the moon. But if the days of creation are regulated by the recurring interchange of light and darkness, they must be regarded not as periods of time of incalculable duration, of years or thousands of years, but as simple earthly days. It is true the morning and evening of the first three days were not produced by the rising and setting of the sun, since the sun was not yet created; but the constantly recurring interchange of light and darkness, which produced day and night upon the earth, cannot for a moment be understood as denoting that the light called forth from the darkness of chaos returned to that darkness again, and thus periodically burst forth and disappeared. The only way in which we can represent it to ourselves, is by supposing that the light called forth by the creative mandate, 'Let there be,' was separated from the dark mass of the earth, and concentrated outside or above the globe, so that the interchange of light and darkness took place as soon as the dark chaotic mass began to rotate, and to assume in the process of creation the form of a spherical body. The time occupied in the first rotations of the earth upon its axis cannot, indeed, be measured by our hour-glass; but even if they were slower at first, and did not attain their present velocity till the completion of our solar system, this would make no essential difference between the first three days and the last three, which were regulated by the rising and setting of the sun.
Keil, C. F., & Delitzsch, F. 2002. Commentary on the Old Testament. (1:31). Hendrickson: Peabody, MA