If it helps, you can think of the IANA time zones as functions from UTC to local time. This is not quite the same as the political definition. For instance, bot

If it helps, you can think of the IANA time zones as functions from UTC to local time. This is not quite the same as the political definition. For instance, both America/Boise and America/Phoenix are in the "Mountain time zone". But when viewed as functions, they're very different -- one is continuous and the other is not. To add another layer of complexity, the tz database is capable of representing time zones whose definitions are themselves changing over time. If your state government decides to switch from Eastern to Central time next year, you still want to be able to accurately answer questions like "what was the current local time in this location at UTC instant t?".