Adam knew the universe was created in six evenings and mornings or he would have told his descendants otherwise.

Genesis is history, not science. Genesis 1 says that Adam was created the sixth day according to Genesis 1. God rested the seventh day. Does it make any sense that God would leave Adam in the dark about the creation time period? There are two reasons in the text that show that Adam would have known the time period – the Sabbath day and Eve.