Understanding Time Zones

The world is divided into twenty-four standard time zones spaced at intervals of 15° longitude. (Technically, due to geographic and political factors, the boundaries of time zones are more elaborate than this. In addition, a few time zones are offset by an odd number of half hours from the Greenwich and U. S. time zones.) Within the confines of each time zone, the hours and minutes of the day are defined in the same way. Time zones were created to eliminate the problem that local noon (defined according to the elevation of the Sun) actually occurs at a different time for nearby towns at slightly different longitudes, so that each town's clocks differed by a few minutes from those of neighboring clocks. This problem was already encountered in making up timetables for long distance train travel, but became completely unmanageable with the advent of modern air travel. Defining time zones means that watches need only be adjusted in one hour increments upon the crossing of each time zone boundary, as opposed to continuously during any east-west journey.

Time zones are usually specified by the number of hours they differ from Greenwich Mean Time. Greenwich, England is defined as the 0 of longitude, and is the center of the Greenwich Time Zone, relative to which other time zones are usually referenced. For example, U. S. Eastern Standard Time (EST) is UT - 5 hours, U. S. Central Standard Time (CST) is UT - 6 hours, U. S. Mountain Standard Time (MST) is UT - 7 hours, and U. S. Pacific Standard Time (PST) is UT - 8 hours.

Almost all time zones differ for an integral number of hours from GMT, but there are a number, the most famous of which in North America is Newfoundland, which differ by an odd number of half-hours. Unfortunately, the system is further complicated by Daylight Savings Time, which is seasonally inserted in some (but not all) time zones. Daylight Savings Time is in effect during the summer months, and is usually one hour ahead of Standard Time. For more information, see Understanding Daylight Savings Time. Therefore, U. S. Eastern Daylight Time (EDT) is UT - 4 hours, U. S. Central Daylight Time (CDT) is UT - 5 hours, U. S. Mountain Daylight Time (MDT) is UT - 6 hours, and U. S. Pacific Daylight Time (PDT) is UT - 7 hours. In the United States, the only states which do not use Daylight Savings Time are Hawaii, Arizona, and most of Indiana. (The situation in Indiana is particularly complicated, because while most of Indiana remains on Eastern Standard Time year-round, some portions near borders maintain the same time as the neighboring state and therefore do shift to Daylight Savings Time.)

A construct related to time zones is the International Date Line at the 180° meridian, which occurs (mostly) in the middle of the time zone offset twelve hours from Greenwich.