When the speed of communication was faster than the rotation of the earth.
But what really made time zones a thing was trains. Everyone basically used their own local "highnoon" as a time reference and it wasn't working for keeping trains on a schedule
ToasterPops t1_jaafamc wrote
Reply to Eli5: When/How did the world realize that it wasn’t the same time everywhere? by Ice_Ice_Fetus
When the speed of communication was faster than the rotation of the earth.
But what really made time zones a thing was trains. Everyone basically used their own local "highnoon" as a time reference and it wasn't working for keeping trains on a schedule
https://www.history.com/this-day-in-history/railroads-create-the-first-time-zones