Time flows inevitably, but the calendar is a human institution--and its history is a colorful mix of science, whim, and pure chance. Ancient peoples recognized that certain natural phenomena (the phases of the moon, the seasons of the year) recurred in a regular pattern. Our earliest record of a firm date comes from Egypt, where the annual rise and fall of the Nile gave a clear marker of the most crucial time of the year: spring planting. Other societies (Hebrews, Greeks, Romans) established years based on lunar cycles or arbitrary counting systems, some of which still survive. But the movements of earth, moon, and sun exist in no simple ratio to one another, and so all these early calendars needed frequent adjustment--with inevitable uncertainty and confusion. At last Julius Caesar scrapped the Roman calendar for one based instead on advanced Alexandrian science, with alternating months of 30 and 31 days, and a leap year to accommodate the odd fraction. His successors almost immediately began tinkering with it, changing the names and the lengths of months; for a while, they even had trouble remembering when to insert leap years. Thirteen centuries after Caesar's reforms, Roger Bacon, an inquisitive English friar, saw that the calendar was still not accurate, and informed the pope of the fact. The Church had downplayed exact measurement of time (why bother when the Second Coming is expected at any moment?) but the fact that Easter was now two weeks distant from the correct date proved a sufficient spur to reform. Two centuries later, the Church accepted Bacon's findings and instituted a new calendar, essentially the one we use today. Veteran science writer and NPR commentator Duncan (Residents: The Perils and Pleasures of Educating Young Doctors, 1996) provides vivid portraits of the various figures who played roles in this process and of their times. A fascinating cross-section of history.