The line of longitude that serves as the reference for measuring time zones is the Prime Meridian. The Prime Meridian is an imaginary line that runs from the North Pole to the South Pole, passing through Greenwich, London, in the United Kingdom. It is designated as 0 degrees longitude and serves as the starting point for measuring east-west distances on the Earth’s surface. Time zones are typically located at 15-degree intervals east and west of the Prime Meridian. As you go eastward from the Prime Meridian, time increases by one hour for each time zone, whereas heading westward reduces time by one hour per zone.