Evening and morning, the first day. Saying creatures lived millions of years in the day only, or the evening only is the real speculation. The Bible says day, it is a day.
3 And God said, “Let there be light,” and there was light. 4 God saw that the light was good, and He separated the light from the darkness. 5 God called the light “day,” and the darkness he called “night.” And there was evening, and there was morning—the first day.