A man named David Rice Atchison was President of the United States for one day and didn't even know it. According to the law at the time, if neither the President nor the Vice President were in office, the President Pro Tem of the Senate (Atchinson) became President. On March 4, 1849, President Polk's term had expired and President-elect Taylor could not yet be sworn in because it was a Sunday. Atchinson did not realize that he had been President for a day until several months later. The law that made Atchinson President for a day has since been changed.