The Cell or Hydrogen? I Don't Know How Are You?

The first person to use a microscope to look at what he called Animacules was Antonivon Leuven-Hook. A Dutch habadesha who ground lenses because he wanted to look at the quality of his curtains that he was buying and selling. But I'm not sure if he would have identified the cell, but that was back in the 16, late 1600s. How long was a day about one and a half billion years ago? Was it a 12 hours, b 18 hours or c 23 hours long?