God created light and darkness – setting them apart as a day
In the beginning of the created universe, God created up the earth, the sun, the moon and the stars we can see at night. His design was the 24 hour day, matching the earth’s rotational period. A single instance of light followed by darkness was a day. “And God said, “Let there be light,” and there was light. And God saw that the light was good. And God separated the light from the darkness. God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day.” (Genesis 1:3–5, ESV) As he continued creating over the next 5 days, he announced the end of each creating period “And there was evening and there was morning, the second (third, then fourth, fifth, and sixth) day.” The usage of day is simple and clear and consistent.
Is a day always a 24 hours elsewhere in the Bible?
The word day consistently refers to a single 24 hour period, except when God does a miracle! Or when the context is a clear figure-of-speech. God made an extended day miraculously when Joshua led Israel in a battle. The sun stayed up and the battle continued after he prayed to God for more daylight to fight the battle. “And the sun stood still, and the moon stopped, until the nation took vengeance on their enemies. Is this not written in the Book of Jashar? The sun stopped in the midst of heaven and did not hurry to set for about a whole day. There has been no day like it before or since, when the Lord heeded the voice of a man, for the Lord fought for Israel.” (Joshua 10:13–14, ESV) Elsewhere, figures of speech like we use such as “a few days” or “just another day” refer to parts of one or more days. We understand them to be ambiguously vague, just as day-oriented figures of speech worked in Bible times.