At what point did humans realize “hey it’s a different time over there than over here”? I feel like this is probably something that would never have been noticed back when things like letters took months to cross the seas.
"Time zones" are a modern invention, but it was understood in antiquity that time was different in different parts of the earth. Here are some previous answers I've given on the subject:
Here are some other old threads on time zones: https://www.reddit.com/r/AskHistorians/comments/4z5u3y/did_people_in_ancient_and_medieval_times_know/
and an overall link drop to clean up anything I missed:
Along with the links provided, it is worth remembering that:
Until the modern era, time was set by the position of the sun ("noon" means directly overhead). So any decent understanding of geometry will tell you that it will vary by longitude on a spherical Earth. This is knowledge the Ancients had.
Time zones as we know them are arbitrary standardizations of those variations, and were only created in modern times to deal with the problems created by technology that allows you to travel very fast (originally trains). They were not things that were "discovered."
The most interesting "discovery" in my mind is that the loss of a day was noted by the crew of Magellan's voyage. This was totally predictable from the geometry but then it came as a surprise and wonder to the people who experienced it (they kept careful track of the calendar because they observed feast days, and were surprised to learn they had missed a day). Even today, with our very good understanding of these things, it still feels somewhat magical and strange when you "lose" days through travel.