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UTC vs GMT vs BST: Why World Time Differences Change

Coordinated Universal Time (UTC) is the international reference time scale used to coordinate standard frequencies and time signals. The International Bureau of Weights and Measures (BIPM) computes it from atomic-clock data contributed by national laboratories. UTC never switches for daylight saving time; Japan Standard Time, for example, remains UTC+9 all year.

Greenwich Mean Time (GMT) is time at the Greenwich meridian and currently shows the same clock value as UTC+0. The terms are often used interchangeably in everyday life, but UTC is an atomic international time scale, while GMT historically derives from mean solar time at Greenwich.

London uses GMT (UTC+0) in winter and British Summer Time (BST, UTC+1) in summer. A London clock being one hour ahead of UTC is therefore correct during BST. New York, Sydney, and many other cities also change their UTC offset seasonally.

A time zone is more than a fixed “UTC plus or minus” number: it is a set of regional rules covering daylight saving time and historical policy changes. Because the US and Europe change clocks on different dates, the difference between their cities can temporarily shift during the year. Always pair a city with the event date when scheduling.

Now for a little timekeeping trivia. Until the mid-19th century, each town set noon by its own local sun. Once railways expanded, travelers had to reset their watches from station to station; US railroads reportedly operated with more than 50 different company times at one point.

The turning point was the 1884 International Meridian Conference in Washington. It selected the meridian through the Greenwich Observatory as longitude zero, laying the foundation for today's global system. Japan adopted Japan Standard Time on January 1, 1888, based on the 135°E meridian near Akashi.

Even atomic time has to account for an unruly planet. Earth's rotation varies slightly, so leap seconds have been inserted to keep UTC close to astronomical time. Because those sudden extra seconds can disrupt computer systems, the international community has agreed to change the current frequent-adjustment system by 2035.