Greenwich Mean Time, or GMT, is a time standard that was established in the 19th century. It is based on the mean solar time at the Royal Observatory in Greenwich, London. GMT serves as a reference point for time zones around the world, helping to coordinate activities across different regions. As the Earth rotates, different parts of the world experience daylight and darkness at different times. GMT helps to standardize timekeeping, so when it’s noon in Greenwich, it’s a specific time in other places, depending on their distance from this prime meridian.