Before 1883, clocks were set to local sun time, with noon marked by the sun’s highest point in the local sky, so each town kept a different time; Chicago was nearly 5 minutes ahead of Rockford and 15 minutes behind Detroit. These differences disrupted railroad schedules. In November 1883, the General Time Convention of Railroad Managers meeting in Chicago established four time zones—Eastern, Central, Mountain, and Pacific—for the continental U.S., each exactly one hour apart. Within each zone, all railroad facilities, operations, and timetables used the same Railroad Standard Time regardless of local sun time at individual stations or towns. Although intended only for railroad use and operations, the system’s clear advantages for commerce led to its nationwide acceptance, and Congress officially adopted it in 1918 through the Standard Time Act.