Greenwich Mean Time is determined by how many degrees of longitude a location or region is from the Greenwich Meridian line. Each 15 degree shift of longtitude from Greenwich Meridian signifies a one-hour difference, giving the Earth 24 different time zones that correspond to the 360 degrees of a circle. Greenwich Mean Time is used as the basis for coordination of time zones around the world.
Greenwich Mean Time has been in use as a global time standard since 1884, at which time it was adopted internationally during the International Meridian Conference. The standard was first informally used by railway companies in England in 1847 but did not become the legal time standard of Great Britain until 1880. Due to the irregularities in Earth's daily rotation patterns, Greenwich Mean Time was replaced as the international time standard in 1972 by the Coordinated Universal Time standard which uses atomic clocks for better accuracy.
Greenwich Mean Time also has some discrepancies due to various economic, social, political and legal factors that causes a difference between geographical and legal times. Legal time zones generally do not follow geographical longitude lines but are slightly shifted to accommodate those countries that are spread through multiple zones.