In practice, if you have participants from different timezones, you agree on one "reference" timezone, which can be also UTC, as in your nomads example. You can consider UTC as just another calendar you can stick to (but you should just not hardcode this in software for this appointment use-case). You actually need a reference to be able to plan an event in the first place. Otherwise you don't know which time you can propose. You would need to propose a "fixed" time for every timezone which participates, but that doesn't work, because these times might not refer to the same physical instant, so the meeting would not be (fully) synchronized. So instead, you take some time from some timezone and translate that to all the other timezones. You can even see that in online games, where some of them have an official "server time", which is globally the same, helping players to meet at the same time instant. UTC itself is another examples of this, used e.g. for global navigation or air traffic control.