Longitude specifies the east-west position of a point on Earth. The prime meridian passes through Greenwich, London at 0° longitude and is used as the standard. India uses 82.5° east as its standard meridian. Time zones group countries or regions that share approximately the same local time, typically differing by whole hours. Local time can be calculated by multiplying the difference in longitude from the Greenwich Meridian by 4 minutes or by taking the westernmost and easternmost longitudes of a country and finding the midpoint.