The world is divided into a number of standard time zones. Roughly speaking, there are 24 time zones spaced at intervals of 15° in longitude. (Practically, due to geographic and political factors, the boundaries of time zones are more circuitous. 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 hour and minute of the day is defined to be the same. Time zones 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 differ 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 than watches need only 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 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 Time (CST) is UT - 6 hours, 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 in North 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 of Standard 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 most of Indiana. (The situation in Indiana is particularly complicated since 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 saving time.)

The following table gives the conversions between universal time (UT) and standard and daylight saving time in the United States.

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.