The Gemara in Sanhedrin relates how the sixth day of creation was a rather long day to say the least. It is difficult to make complete sense of this explanation. But it could possibly prove that we are not sure how long a day lasted at the beginning of creation.
The Gemara relates the following regarding day six:
The first hour, Hashem took dust to form man. During the second hour, Adam grew. In the third hour, he was given veins and sinews to complete his form. He was given a נשמה in the fourth hour. In the fifth hour, he stood on his feet. In the sixth, he gave names to the animals. In the seventh, he was joined by Eve. In the eighth, she gave birth to Cain and Abel. In the tenth, they were commanded to stay away from the forbidden trees. In the eleventh, they sinned. And in the twelfth, they were judged and sent out of the garden.
As is the case with Midrashim, we are to glean whatever we can. I thought this was an unusual comment worth sharing.