To alleviate this concern, time is now calculated using atomic clocks such as hydrogen masers and cutting-edge "cesium/caesium fountain" clocks, which will not gain or lose a second of time in 100 million years. While a discrepancy of a few seconds a day made little difference 100 years ago, in the modern era even such a small variation can have a negative impact on sensitive scientific and technical operations. However, as technology progressed, scientists realized that the speed of Earth's rotation is not constant, but fluctuates by fractions of a second at both predictable and unpredictable intervals. When time zones were first introduced, the time was calculated and timekeeping devices were calibrated by measuring the movement of astronomical bodies-particularly the rotation of the Earth. ![]() The more correct label for time zones is Coordinated Universal Time, or UTC. The term GMT is often used incorrectly, especially among English speakers, as it technically refers only to the time zone in which Greenwich, England is located (GMT+00:00). Greenwich Mean Time (GMT) compared to Coordinated Universal Time (UTC) For example, the countries of Turkmenistan, Iran, and Oman are geographically positioned in a vertical north-south row, but they adhere to three different time zones (including Iran's custom GMT+03.50). Countries whose territory touches more than one time zone are free to choose which of those time zones they wish to utilize. While the borders between time zones are clearly defined, they are also notably flexible. Time zones are named according to their distance from the longitudinal meridian that passes through Greenwich, England and establishes the GMT+0:00 time zone. GMT divides the world into 26 time zones-one for each hour in the day plus two overlapping extras to accommodate the remote Pacific Island country Kiribati-each running vertically from Earth's north pole to its south pole. Originally developed by British railroads in the 1840s and made a global standard at Washington DC's International Meridian Conference in 1884, Greenwich Mean Time established a universal international standard time system that enabled the coordination of time all around the world. ![]() The Importance of Greenwich Mean Time (GMT) Countries in the GMT+08:00/UTC+08:00 Time Zone: Thus, China's full population falls into UTC+08:00. Although China's geographical area stretches across five time zones, the country's government has chosen to use only GMT+8:00 across its entire expanse. This is primarily due to an unusual decision by the government of China, one of the two most populous countries in the world (along with India). GMT+08:00/UTC+08:00 has a larger population than any other time zone in the world, and is home to an estimated 24% of the Earth's inhabitants. Located in Eastern Asia, UTC+08:00 is one of the earliest time zones in the world (UTC+12:00 being the earliest). GMT+8 also has several region-specific alternate names, including China Taiwan Time (CCT), Western Standard Time ( Australia), Hong Kong Time, Irkutsk Time, and more. As such, the GMT+8:00 time zone is also known (more correctly) as UTC+08:00. GMT has actually been replaced by Coordinated Universal Time (UTC), although GMT remains the more commonly known of the two terms. Next time change is highlighted.GMT+8 countries are countries which lie partially or completely within the GMT+08:00 time zone, where the local time is eight hours ahead of Greenwich Mean Time (GMT+00:00), which is the time in Greenwich, England. Time Changes in London Over the Yearsĭaylight Saving Time (DST) changes do not necessarily occur on the same date every year. Some other countries, such as Ireland, observe the same local time, but under a different name. The UK is the only country that observes BST in the summer. What About BST?ĭuring Daylight Saving Time, clocks in the United Kingdom are set forward one hour to British Summer Time (BST). Until 1972, time zones were defined by their offset from GMT based on mean solar time at the prime meridian in Greenwich near London. GMT is a time zone used in the UK and other European and African countries.Ī source of confusion is that GMT used to be the world's time standard before UTC was introduced.Since GMT has a UTC offset of zero, there is no time difference between the two. Time zones are defined by their offsets from UTC. UTC is a time standard that forms the base of time zones worldwide and is not used as a time zone in any country.However, there is an essential difference between the two: In practice, GMT and UTC share the same time on a clock, which can cause them to be interchanged or confused. London uses Greenwich Mean Time (GMT) during standard time and British Summer Time (BST) during Daylight Saving Time (DST), or summer time. The graph above illustrates clock changes in London during 2024. ![]() Business Date to Date (exclude holidays).
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