Some countries advance their clocks in spring by one hour and set it back in autumn. Moreover, the time zone boundaries may change over time due to state borders changes or government decisions. Time zone boundaries are quite complex because they often reflect country or district borders. The table below shows the zones with non-integer offset: Time zone offsets and boundaries.Ī time zone offset is generally an integer number of hours from -12 to +14, but few territories have the time offset rounded to 30 minutes or even 15 minutes. Modern time zone time offsets are counted from UTC, e.g., UTC+03, UTC-06. In the 20th century, Greenwich Mean Time (GMT) was replaced by more accurate Coordinated Universal Time (UTC), which is counted by an atomic clock and corrected manually from time to time to make it closer to the based on Earth's rotation universal time on the zero degree meridian. The process of introducing GMT-based time in the world lasted for decades. During International Meridian Conference, which is held in the United States in 1884, most of the participated countries have chosen the Greenwich meridian as an initial meridian for longitude and decided " to adopt a universal day for all purposes for which it may be found convenient, which is to begin for all the world at the moment of mean midnight of the initial meridian, coinciding with the beginning of the civil day and date of that meridian This universal day is to be a mean solar day and is to be counted from zero up to twenty-four hours". Worldwide navigation was another problem that demanded single world time. It was hard for railway companies to make different timetables in local times for every town on the route.
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