UTC or unix timestamp gets you moment of time. You know exactly moment event happened. Definitions can change, but usually not retroactively. TZdata is a databa

UTC or unix timestamp gets you moment of time. You know exactly moment event happened. Definitions can change, but usually not retroactively. TZdata is a database of such changes, and stores definition changes, basically all *nix OSes depend on it to cast moment of time into timezone. You can always convert timestamp in the past into desired timezone. If you work with calendars and want you know what "04:00 tomorrow" means - good luck, you'll need it. as example, "04:00 tomorrow" may not exist at all