The world is divided into a number of standard time zones. Roughly speaking, there are 24 time zones spaced at intervals of15° in longitude. (Practically, due to geographic and political factors, the boundaries of time zones are morecircuitous. In addition, a few time zones are offset by an odd number of half hours from the Greenwich and U.S. timezones.) Within the confines of each time zone, the hour and minute of the day is defined to be the same. Time zoneseliminate the problem that local noon (defined according to the elevation of the Sun) actually occurs at a differenttime for nearby towns at slightly different longitudes, so that each town's clocks differ by a few minutes fromthose 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 than watches needonly be adjusted in one hour steps upon crossing of a time zone boundary, as opposed to continuously along any east-west journey.
Time zones are usually specified by the number of hours they differ from Greenwich mean time. Greenwich, England isdefined as the 0 of longitude, and is the center of the Greenwich time zone, relative to which other time zones areusually referenced. For example, U.S. Eastern Standard Time (EST) is UT - 5 hours, U.S. Central Time (CST) is UT - 6hours, U.S. Mountain Time (MST) is UT - 7 hours, and U.S. Pacific Time (PST) is UT - 8 hours.
Almost all time zones differ an integral number of hours from GMT, but there are a number, the most famous of which inNorth America is Newfoundland, which differ by an odd number of half-hours. Other examples are Iran, Afghanistan, India,Nepal, Myanmar, and central Australia.
Unfortunately, the system is further complicated by daylight saving time, which is seasonally inserted in some(but not all) time zones. Daylight saving time is in effect during summer, and is usually one hour ahead ofStandard 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 saving time are Hawaii, Arizona, and mostof Indiana. (The situation in Indiana is particularly complicated since while most of Indiana remains on EasternStandard Time year-round, some portions near borders maintain the same time as the neighboring state and therefore do shift to daylight saving time.)
The following table gives the conversions between universal time (UT) and standard anddaylight saving time in the United States.
| standard time zone | hours to add to UT | daylight saving time zone | hours to add to UT |
| Eastern standard time (EST) | -5 | Eastern daylight time (EDT) | -4 |
| Central standard time (CST) | -6 | Central daylight time (CDT) | -5 |
| Mountain standard time (MST) | -7 | Mountain daylight time (MDT) | -6 |
| Pacific standard time (PST) | -8 | Pacific daylight time (PDT) | -7 |
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 12 hours from Greenwich.
Coordinated Universal Time, Daylight Saving Time, Ephemeris Time, Greenwich Mean Time, Standard Time, Time, Universal Time
HiLink Communications. "Local Times Around the World." http://www.hilink.com.au/times/.
NIST and USNO. "The Official U.S. Time." http://www.time.gov/.
Seidelmann, P.K. (Ed.). "Time Zones." §2.27 in Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books, pp.56-58, 1992.
Stewart, P. "Time Zone Converter." http://poisson.ecse.rpi.edu/cgi-bin/tzconvert.
United States Naval Observatory. "World Times Zones." http://tycho.usno.navy.mil/tzones.html.
© 1996-2007 Eric W. Weisstein
