Yohm is used in many contexts that reveal a long or indefinite period consistently throught Genesis, and the OT.
For instance, Genesis 2:4
"THis is the account of the heavens and the earth when they were created in the day (yom) that the Lord God made the earth and heaven."
Here we see a clear example of how 'yom' was used in early Genesis to refer to a period of time for the entire creation event.
Not also that in ancient hebrew, there was no word other than 'yom', to represent ideas such as: epoch, era, generation, or other long period of time. So when you see things like "in the generations of our fathers" it uses 'yom'.
The other words in Genesis when it refers to "morning and evening" these words are "boger, and ereb", and they also can mean "the closing of a day, or era or epoch" or "the beginning of a day, era or epoch".
Don't be confused by what YEC's force upon scripture. This does not imply a 24hr creation day in the slightest.