How the border between time zones is established?

Answer 1

Since each country determines what its time zone (offset from UTC) will be and if and when to observe Daylight Saving Time/ Summer Time, they are almost always political borders. However, within countries that cover enough longitude to have multiple time zones, like the United States, where people work or get most of their radio and local TV stations from has a lot to do with it. For example, a lot of people who live near the northwest corner of Indiana work in Chicago, so five or six counties in that corner of Indiana are on Central Time. Sometimes they use a sparsely populated part of a state to separate time zones. I've noticed, however, that the extent to which the U. S. and Canada have multiple time zones within states and even within counties is more the exception than the rule. Places like Mexico, Brazil, Australia (with one exception), Russia, Mongolia, Kazakhstan and Indonesia do not split individual states, provinces, oblasts, or whathaveyou into multiple time zones (although there are ten Mexican cities along the U. S. border that start and end Daylight Saving Time on the same dates as the U. S., while the rest of their states change their clocks on the same dates as the rest of Mexico).