What is a Time Zone?
A time zone is a region on Earth that uses a uniform standard time for legal, commercial, and social purposes. They are often based on the boundaries of countries or lines of longitude.
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory located in Greenwich, London, considered to be located at a longitude of zero degrees. Although GMT and Coordinated Universal Time (UTC) essentially reflect the same time, GMT is a time zone, while UTC is a time standard that is used as a basis for civil time and time zones worldwide. Although GMT used to be a time standard, it is now mainly used as the time zone for certain countries in Africa and Western Europe. UTC, which is based on highly precise atomic clocks and the Earth's rotation, is the new standard of today.
UTC is not dependent on daylight saving time (DST), though some countries switch between time zones during their DST period, such as the United Kingdom using British Summer Time in the summer months.
Most time zones that are on land are offset from UTC. Generally, time zones are defined as + or - an integer number of hours in relation to UTC; for example, UTC-05:00, UTC+08:00, and so on. UTC offsets can range from UTC-12:00 to UTC+14:00. Most commonly, UTC is offset by an hour, but in some cases, the offset can be a half-hour or quarter-hour, such as in the case of India (UTC+05:30) and Nepal (UTC+05:45).
World Time Zones
Time zones throughout the world vary, and used to vary even more than they currently do. It wasn't until 1929 before most countries adopted hourly time zones. Nepal, the final holdout, did not adopt a standard offset of UTC until 1956.
Generally, a time change of 1 hour is required with each 15° change of longitude, but this does not necessarily always happen. For example, China and India only use a single time zone even though they are countries that encompass a far larger area than 15° of longitude. Russia, on the other hand, is divided into 11 time zones.
U.S. Time Zones
Many countries have more than one time zone. In the U.S., there are a total of 9 time zones used. The most well-known include the Eastern, Central, Mountain, and Pacific Time Zones. These time zones encompass most of the contiguous United States.
- Eastern Time Zone (ET): UTC-05:00 – Includes 22 states in the eastern part of the contiguous United States, including some parts of Canada, Mexico, Panama, and the Caribbean islands.
- Central Time Zone (CT): UTC-06:00 – Includes parts of Canada, the U.S., Mexico, Central America, and some Caribbean islands. Only 9 states are fully within the Central Time Zone.
- Mountain Time Zone (MT): UTC-07:00 – Includes parts of Canada, Mexico, and the U.S. 5 states are fully within the MT zone.
- Pacific Time Zone (PT): UTC-08:00 – Includes parts of Canada, the western U.S., and western Mexico. California and Washington are fully within PT.
The Atlantic, Alaska, Hawaii-Aleutian, Samoa, and Chamorro Time Zones cover the rest of the United States, including Alaska, Hawaii, and other U.S. territories.
Daylight Saving Time (DST)
Many countries or regions, especially those in Europe and North America, practice daylight saving time (DST), sometimes referred to as "summer time," with the intent of saving energy. This typically involves advancing the time on a clock by one hour in early spring, and "rolling back" the time in autumn. Manipulating the time as the seasons change can better synchronize working hours with sunrise and sunset times in certain areas.
* Note: This Time Zone Calculator performs standard UTC offset calculations and does not automatically account for historical or local Daylight Saving Time changes.