Chicago, United States · UTC-5 ☆

Chicago runs on America/Chicago: UTC-6 in winter (Central Standard Time), UTC-5 in summer (Central Daylight Time). The second Sunday in March and the first Sunday in November mark the DST transitions.

Chicago has a specific claim on time standardization history. In 1883, the American railway industry, working through the General Time Convention meeting in Chicago, adopted four standard time zones for North American railroads. The plan, developed by William F. Allen, replaced the dozens of local times maintained by different cities and railways. The new system took effect on November 18, 1883, a day called “The Day of Two Noons” because many cities experienced noon twice as their clocks were adjusted.

The day of two noons

Before standardization, every city kept its own solar time. Chicago’s local noon came 9 minutes and 32 seconds after New York’s noon, and 7 minutes and 56 seconds before St. Louis’s noon. This created impossible chaos for railway timetables. When the railways adopted standard zones, cities briefly had two noons: their old local noon and the new standard noon. Newspapers covered the transition as a major event.

Chicago, as the continent’s railway hub, was at the center of this change. The city’s Board of Trade building had its clock adjusted at the moment of the transition. Congress did not formally adopt the system until 1918, but the railroads’ practical implementation made it effective from 1883.

The trading floor clock

Chicago’s financial identity is built around the commodity and futures exchanges: the CME Group (Chicago Mercantile Exchange and Chicago Board of Trade), now one of the world’s largest derivatives exchanges. The trading floor opens at 8:30am CT during agricultural trading sessions. London, New York, Tokyo all coordinate around Chicago’s agricultural market hours.

The time conversion from Chicago to London puts London six hours ahead in winter, five in summer. Brandon in Canada’s prairies is on the same clock.

Sources

• IANA Time Zone Database
• CME Group
• Bartky, Ian R. Selling the True Time: Nineteenth-Century Timekeeping in America. Stanford University Press, 2000.