Common time, often referred to as standard time, is a system used to synchronize clocks across different regions. It divides the world into time zones, each typically one hour apart, based on the position of the Earth relative to the Sun. This system helps coordinate activities, travel, and communication across various locations. The concept of common time emerged in the 19th century, largely due to the expansion of railroads and the need for consistent scheduling. Before this, local mean time was used, which varied from place to place. The establishment of Greenwich Mean Time (GMT) as a reference point helped standardize timekeeping globally.