"It does not say that it was night time and it was day time which made one day; but it says 'it was evening,' which means that the period of the day time came to an end and the light disappeared. And when it says 'it was morning,' it means that the period of the night time came to an end and the morning dawned. Then one whole day was completed."
Source: "The Sabbath in Jewish Ritual and Folklore," in the "Rabbinic Essays by Jacob Z. Lauterbach," Hebrew Union College Press, Cincinnati, 1951, p.446-447
The unedited full-text of the 1906 Jewish Encyclopedia - SAMUEL B. MEÏR (RaSHBaM)