As the Earth rotates through 1 day, it moves about 1 along its orbit around the Sun. Telegraph lines linked many widely separated longitudes instantaneously, but needed to coordinate them. Small differences in local solar time began to matter. Each time zone keeps local solar time for a fixed reference longitude. Cities, counties, and small countries want to be on the same time system for ease of governance.
Some states refuse to have multiple time zones.
Keep some island nations from being divided. This results in irregular time zone boundaries that cannot be easily computed a priori. One usually has to resort to consulting a map. In my opinion, the single best site on time and timesystems on the internet is the Time Service Department of the U.S. Naval Observatory . 2001 January 12 Copyright Richard W. Pogge, All Rights Reserved.