The International Date Line, abbreviated as the IDL, is an imaginary line which runs roughly along the 180 degree line of longitude, in the middle of the Pacific Ocean. International convention accepts the International Date Line as the location where one day rolls over to the next, with the area east of the International Date Line one day ahead of the west. The line is necessary to address certain oddities which occur during travel: people going all the way around the world perceive themselves either gaining or losing a day, depending on which direction they traveled in, as happened to Magellan on his first voyage.

The world is divided up into 24 time zones, one for each hour of the day. The time zones are designed so that local noon falls when the sun is roughly overhead. Traveling west, you lose an hour for every 15 degrees of longitude, while traveling east, towards the sun, you gain an hour. The time zones are described in terms of their relationship to the prime meridian, which falls at 0 degrees of longitude in Greenwich, England. If someone was 45 degrees away from Greenwich, they would experience a three hour time difference: 45 degrees to the west would mean that the person was in Greenwich Mean Time (GMT) -3, while someone 45 degrees to the east would be in GMT +3. The International Date Line falls directly opposite the Prime Meridian in the time zones GMT + 12 and GMT -12.

The location of the International Date Line was chosen because it is in a relatively uninhabited area of the Pacific. However, small deviations did need to be made in the line to ensure that residents of areas like Alaska and Russia could all operate within the same day during business hours: otherwise, administrative chaos would result. The small zigzags in the line keep things running smoothly on a daily basis within nations that would otherwise be in a state of confusion. Crossing the International Date Line means that you immediately add 24 hours to your watch if traveling east, or subtract a day when going west.

Traveling across the International Date Line can be confusing, especially when you are taking a short trip, for example from Fiji to Hawaii. You would end up arriving in Hawaii before you had left Fiji, because Fiji is a day ahead of Hawaii, and two hours behind: when it is noon in Hawaii, it is 10:00 AM in Fiji on the next day. Confusions of days and schedules do sometimes lead to mishaps, but most airlines and travel agencies keep the International Date Line in mind when informing travelers about schedules, expected arrival times, and itineraries.