Yom is the Hebrew word for day, and is used as such throughout the Bible. In most cases, there is no question of the meaning of the word due to the surrounding context. However, when it comes to the creation of the universe, or anything really, we don't have any context at all unless we know two things; the thing that is being created, and the process that is being used to create it. These are two things that we really do not have an understanding of when we look at the book of Genesis. We do not know what the universe is, and we most definitely do not know, with complete certainty, the process by which it was created.