Contrary to what its title suggests, this book is not a history of clock design, manufacture, or marketing. It is instead about the rise and fall of commercial time signal distribution by astronomical observatories in the nineteenth-century United States—time-telling rather than timekeeping. It narrates complex interactions of scientific, business, and governmental establishments around the activity of telling any buyer of the service, and therefore also the public at large, what time it was, accurately and consistently. It also discusses the adoption, late in the century, of American standard time zones and the eventual international agreement to accept the location of Greenwich in England as 0°, the prime meridian from which other meridians were calibrated.