When the Book of Genesis was written, was a year 365 days?
Since Moses wrote the book of Genesis, at that moment in history a year was a constant. Moses was probably thinking in terms of 360 days composing a year. This would mean that a “day” was probably a literal 24 hour period.
Why 360 instead of 365? The Jewish calendar counted days for a year differently. There were only 360 days in the year according to that calendar. It is the Roman calendar that gives us 365 days in a year.
It has been corrected in human history at some point to reflect the fact that one quarter of the day is missing every four years. This is why they created the leap year, to correct the Roman calendar.
But the Jewish calendar is different. It’s most likely that Moses would’ve thought more of the Jewish calendar because the Romans didn’t exist in his time. If you are referring to the literary approach of the book of Genesis, that is a different matter.
There are many scholars who take “day” in the book of Genesis, especially the first chapter, to mean a season or era instead of a literal 24 hour day. They use this to understand that there was much more time than can be counted using the genealogies of Genesis to form an understanding of how old the earth is.
However, I contend that the “day” of Genesis 1 and the entire book means a 24 hour period. No one can count how many years the Earth has existed from the biblical genealogies alone. We know for a fact based on the other accounts of genealogies from the same time that Moses records them contain people not mentioned in the Genesis records.
This means that these records are selective, as are all the genealogies in the Bible, and cannot be counted on to give us a complete count of the years in them. The writers of the books of the Bible are using genealogies to make a theological point, not tell us how old the earth is or how long the time is from the beginning of the earth.