This is because they are larger and flow into several time zones. For example, in the United States, there are six time zones.
Because they range through such a broad range of longitudes (east-west extent) that it takes the sun considerably more than one hour to cross the whole country.
Countries adopt multiple standard times mainly due to their large geographical size and the differences in solar time caused by their range of longitudes. For example, the United States and Russia each use several time zones to manage these variations. Additionally, decisions may also be influenced by political and cultural factors.
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