Constantine was a flaming Antisemitic . He issued an edict, forbidding Christ followers to celebrate any Christian festival on the same day that a Jewish holy day was kept. That included the 7th day Sabbath. Constantine is the one who reconfigured the Sabbath (to Sunday) - the time it started and the time it ended.
Genesis 1:5 “God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day.” (NRS)
Evening is mentioned FIRST. First comes evening - the darker part. Then comes morning - the lighter part - of the repeating 24 hour cycle.
Within one 24 hr period, there is only 1 evening, and 1 morning. This is what we see repeated after each of the first six days. Why would we conclude that the 7th day - the one especially blessed forever - was somehow a different time period, with a different beginning and ending point, from the first six days?
What's so difficult?
The captives who returned from Babylon were very anxious to keep God's Law, so as to avoid calamity in the future. Why would their teachers have changed the commands. That would be to invite God's disfavor. That doesn't make sense.