When the North American railroads adopted standardized time in 1883, they were not responding to a public clamor for convenience. Before that November, most towns kept their own local time, set by the sun's position at noon, so that a clock in one city might differ by several minutes from a clock forty miles east. For ordinary residents, these discrepancies caused little trouble; a farmer had no reason to know the hour in a town he would never visit. The railroads, however, ran on schedules, and schedules assembled from dozens of incompatible local times produced a tangle no timetable could resolve. By imposing a grid of uniform zones, the companies solved a problem that was, at root, their own. Yet the reform outlived its narrow origin. Once travelers grew used to consulting a single authoritative clock, the older habit of trusting the local sun came to seem quaint, even backward. Within a generation, standardized time had migrated from the timetable into the courthouse, the factory, and the schoolroom. What began as a tool for coordinating trains had quietly redefined what an ordinary person meant by the word 'noon.'