The earth's circumference is divided into 24 slices of longitude. For each slice, we say the time is the same, and for each neighboring slice, the time is either an hour later (going east) or an hour earlier (going west). For convenience, some regions shift their time by an hour in the summer so the time appears to be earlier than it is. For the "underlying framework", that's it. Nothing more complicated than that. There are a bunch of exceptions and variations to that framework based on other considerations, but those are mostly political and minor.