Most theologians throughout church history agree that in using the phrase “the evening and the morning were the first day,” the Scriptures are speaking of a literal 24-hour day, rather than a period of years.
“To understand the meaning of ‘day’ in Genesis 1, we need to determine how the Hebrew word for ‘day,’ yom, is used in the context of Scripture...A number, and the phrase ‘evening and morning,’ are used for each of the six days of creation (Genesis 1:5,8,13,19,23,31). Outside Genesis 1, yom is used with a number 410 times, and each time it means an ordinary day—why would Genesis 1 be the exception? Outside Genesis 1, yom is used with the word ‘evening’ or ‘morning’ 23 times. ‘Evening’ and ‘morning’ appear in association, but without yom, 38 times. All 61 times the text refers to an ordinary day—why would Genesis 1 be the exception? In Genesis 1:5, yomoccurs in context with the word ‘night.’ Outside of Genesis 1, ‘night’ is used with yom 53 times—and each time it means an ordinary day. Why would Genesis 1 be the exception? Even the usage of the word ‘light’ with yom in this passage determines the meaning as ordinary day.” Ken Ham, et al., The Answers Book (revised and expanded)