About world time
Before the world time had been established the actual local time was used, that is the time a sun-dial would show, having noon at the time of sun transit.
With the development of modern society and increase of mobility defining a coordinated time system became crucial. This was achieved with the Greenwich mean time (GMT). The globe was divided in 24 timezones with an offset of one hour to each other. The mean solar time at Royal Observatory in Greenwich near London was determined as the origin, all timezones are defined by the offset to the Greenwich mean time. In 1972 the Coordinated universal time (UTC) replaced the GMT as a time standard. The UTC is derived from the
International Atomic Time being based on atomic clocks rather than astronomical observations, which were the basis for GMT.