Official local time of a region or country.
Local mean solar time depends on longitude; it advances by four minutes per degree eastward. The Earth can thus be divided into 24 standard time zones, each approximately 15k in longitude. The actual boundaries of each time zone are determined by local authorities and in many places deviate considerably from 15k. The times in different zones usually differ by an integral number of hours; minutes and seconds are the same. See also Greenwich Mean Time .