Evening and morning, that is, beginning and ending, is the beginning and ending of whatever "Yom" means. Genesis, chapter one, records six creative "yoms".
One step in determining the meaning of a word, such as "yom", is to see how Scripture uses the word in other places. I understand that Hebrew has only about 8600 words available. English has about half a million words available. Therefore Hebrew has to use the same word for more than one meaning even more often than English does.
So the question is, "How is 'Yom' used in other parts of the Pentateuch, all of which are the writings of Moses, and are inspired by God?"
As it turns out, "Yom" is used in several ways by Moses and by the other Old Testament writers. It is used for daytime, that is for 12 hours, it is also used as a 24 hour period, it is used for a prophetic year, it is used for an age, it is used for a period of time, and it is used for an eternity. So which of these periods of time does God mean?
From this at least we know that "Yom", in Genesis chapter one, stands for "a period of time", but for WHICH period of time is unclear.
Notice also that the six "Yoms" of creation all begin with the words "And God said", "And God said" , "And God said". It doesn't say, "And God wrote", "And God wrote", "And God wrote". God SAID things. Man WROTE them down. Sometimes fallible man misinterpreted what God said when he or she wrote them down or translated them. For example, in the King James Version, in the famous "love" chapter of 1 Corinthians 13, the key word in the chapter is translated "charity" rather than "love". But now we all know now that the word "love" is a far better translation than the word, "charity". Let's be careful that we do not translate the word, "yom" in a way that does not accurately reflect what God "said".
Please note also, that the first "Yom" of creation "And God said", doesn't begin until Genesis 1, verse 3. That leaves Genesis 1:1-2 to describe what happened "In the Beginning" and prior to the first "Yom" which starts with the first "And God said" in verse 3.
Then also, let us remember that God gave us TWO declarations of Creation, and they cannot disagree. They must agree since they both are inspired by God. Only OUR understandings of them might disagree. So if there is an apparent disagreement, it is OUR mistake, not God's mistake. We have only misunderstood or misinterpreted what God said or did whenever there is an apparent disagreement.
That is to say, not only does THE SCRIPTURE DECLARE what God said, but Psalms 19:1-2 says that "THE HEAVENS DECLARE the glory of God; THE SKIES proclaim the work of His hands. Day after day they pour forth speech; night after night they reveal knowledge." NIV
A good example of progress over the years in this area can be found in the March 2014 issue of the National Geographic. The article, called "The Truth About Black Holes" illustrates that though there is much progress in the understanding of the heavens, they are still a long way from being complete. What may thought to be understood about the heavens now, may change drastically in the near future. So it is not surprising that there are currently still apparent contradictions between the interpretations of "The Scriptures" and the interpretations of "The Heavens".
Also, what may be unclear from ONE of the declarations of God may be clarified by the OTHER declaration of God. These two declarations cannot contradict each other. Again, only OUR interpretations and understandings of the two can contradict each other. When they disagree, we need to reassess our interpretations and understandings of these two inspired declarations of God, "The Scriptures", and "The Heavens".
Hopefully these thoughts will help to get us on the right track in our understanding and interpretation of what God says in Genesis, chapter one.
What God said, as recorded in Scripture, was written down when scientists in those days believed that the universe consisted of four elements, fire, air, water, and earth. Look now how far that the scientists have come! In many ways they are getting closer and closer to what the Scriptures declared thousands of years ago.
And also, with the recent studies on the Dead Sea Scrolls and other scholarly work, the understanding of Scripture is also getting closer and closer to what God actually said, as well.
Some day the two will be completely unified, either before the Lord returns, or more likely, thereafter...