The author of Genesis 1 (Moses) did not use the definite article before the word day on the first through fifth days. The article is normally used to make the following noun definite. Therefore, since Moses did not use the article here, Yom does not necessarily mean a definite period of time and the first five days of creation may have been ages.

Response:
The definite article is usually used in Hebrew to make the noun definite (ball vs. the ball), however, there are exceptions. Hebrew authors used words very carefully. According to Stambaugh, the "most common observation among Jewish and Christian commentators is that the use of the article on the last two days is to show the importance of the sixth and seventh days" (1988. ICR Impact Article 184, The Meaning of "Day" in Genesis).

In addition, the combination of the noun 'day' with the preceding number, 'first', 'second', 'third,' etc. denotes a definite noun because numbers in Hebrew have a definitive quality in themselves. There are 13 other occurrences in the Old Testament where the noun does not have an article, but is definite because it is with a number.

The absence of an article with the word 'day' on the first five days of creation does not mean that the days are long periods of time.