Many towns and cities around the world used to set clocks based on observing the sun and the stars. This occurred prior to the late 19th century. Dawn and dusk occur at different times at different places because of the Earth’s rotation. However, time differences between distant locations were barely noticeable because of long travel times and the lack of long-distance communications. The expansion of transport and communications, as well as trade globalization, during the 19th century created a need for a more unified time-keeping system.
Moreover, various meridians were also used for longitudinal reference among different countries. The Greenwich Meridian was adopted in 1884 as the initial or prime meridian for longitude and timekeeping. Read timeanddate.com’s article on the history of time zones for more detailed information on why we have time zones today.