24 time zones and many more subdivisions
Standard time is the time of a town, region or country, established by law or general usage as civil time. It is determined locally.
For example, the whole of China, one of the largest countries in the world, has decided to adopt a single time zone.
The concept of standard time was adopted in the late 19th century in an attempt to end the confusion that was caused by each community's use of its own solar time. Some such standard became increasingly necessary with the development of rapid railway systems and the consequent confusion of schedules that used scores of different local times kept in separate communities. (Local time varies continuously with change in longitude.)
The need for a standard time was felt most particularly in the United States and Canada, where several extensive railway routes passed through places that differed by several hours in local time.
Sir Sandford Fleming, a Canadian railway planner and engineer, outlined a plan for worldwide standard time in the late 1870s. Following this initiative, in 1884 delegates from 27 nations met in Washington, D.C., for the Meridian Conference and agreed on a system basically the same as that now in use.
The present system employs 24 standard meridians of longitude (lines running from the North Pole to the South, at right angles to the Equator) 15º apart, starting with the Prime Meridian through Greenwich, England . These meridians are theoretically the centres of 24 standard time zones; in practice, the zones have in many cases been subdivided or altered in shape for the convenience of inhabitants. https://greenwichmeantime.com/info/time-zones-history/
Standard time also fits in with the discussion on radio. The telegraph is also involved.
Basically, Western Union began using the telegraph to send out time signals, synchronized to the U.S. Naval Observatory, almost immediately after the conclusion of the War between the States. These signals went to telegraph offices, railroad stations, and in some cases, city halls, clock shops, and other publicly displayed clocks. This made it relatively easy for a gentleman, or lady, to set their personal timepiece accurately, and keep their home clocks accurate, as well. A description of how it worked can be found here:
As for the radio connection, one of the benefits of global wireless broadcasting, predicted by no less than Nikola Tesla, was "accurate time". This is something which most people reading this probably take for granted, today. Cellphones (radio transceivers), and GPS receivers, all display the time, synchronized to highly accurate, standard clocks. Television and radio stations all get their time from the same sources. One can purchase clocks, and even wristwatches that automatically calibrate themselves to a radio signal each day. The same can be done for computers.
Technically, it's called "coordinated universal time", today. But I still prefer GMT. I also continue to call the organization that provides time signals in the United States the National Bureau of Standards (NBS). At some point, some dufus got it changed to 'The National Institute of Standards and Technology'. Yucgh!
Time zones can also be referred to by letter (or phonetic), as described here:
with GMT being Z, or Zulu time.
I was half way thinking about starting a discussion here for telegraphs. You are welcome to run with that. Of course I would need to bring up the 1859 Carrington Event.