Chicago had long been a transportation hub, and the country's some three hundred time zones -- in addition to the fact that most cities determined local time on their own -- had become a problem in scheduling trains.
Timezones were established in Europe a few decades prior, but the US railroads were reluctant to change. The change was made official on November 18, 1883, about a month after their meeting. A signal, marking the exact time, was sent to the major railroads via telegraph.
(Not to be confused with Daylight Savings, an idea which Ben Franklin had while in Paris in 1784.)