GMT is the clock time recorded at the Royal Observatory in Greenwich, London. GMT is not affected by Summer Time (daylight saving time) when clocks change in the UK twice a year. When the sun reaches its highest point precisely above the Prime Meridian, it’s 1200 noon in Greenwich.GMT is a useful reference when trading financial markets. It’s essential to know what times various markets open worldwide relevant to your local time. For example, in a 24hr market such as forex trading, knowing the opening times of the different trading sessions in Sydney, Tokyo, London, and New York when based in Europe could prove valuable.