system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are time zones a thing? Why not just have a global time and some places the sun is up at 12am and down at 12pm? Why must (generally) the sun be up 6am to 6pm?
gpt
Across most of human history, time was always determined by the apparent local time.  Noon was when the sun was highest in the sky & there was no complication about it.  Since people had to walk, take horses or get on a boat to get anywhere, this was adequate.

...then we developed telegraphs & trains.  Suddenly we had instant communication across vast distances and giant machines that needed to be synchronized so they didn't ram into each other & cause massive destruction.

Time zones were a compromise between our historical use of time being relative to the sun & the need to have a standardized time so people could coordinate across large distances.

If you want just one time, you're free to use UTC (time-zone 0) for everything.  Many multinational companies, militaries and computer database programmers use it to avoid the confusion of timezones & the headaches of daylight savings times.

China has taken the approach of a single unifying time zone across the whole country.  It would cover 5 standard time zones but they put the whole country on the same time as the capital.