The International Date Line is an imaginary line of navigation on the surface of the earth running between the North Pole and the South Pole to demarcate a change from one calendar day to another. It passes across the middle of the Pacific Ocean approximately along the 180th degree of longitude, but deviating at various points to include overlapping areas governed by some island groups and territories to maintain internal uniformity. It is located halfway around the globe from the Prime (Greenwich) Meridian at 0 degrees longitude. Traveling from east to west, the local time gains one hour (clock set back one hour) for each 15 degrees of longitude traveled. This would amount to 24 hours gained for one circumnavigation, unless the calendar date was changed one day forward upon crossing the halfway point. Likewise, when traveling west to east, the calendar date is changed one day backward when crossing the date line. Seabourn voyage itineraries are based upon days actually spent on board, and the dates are noted for convenience, including days lost or gained in crossing the International Date Line. Departure days and disembarkation days are always quoted in local time and date.