The International Date Line

The International Date Line is the place on Earth where one calendar day ends and the next begins. It runs from pole to pole through the Pacific Ocean, roughly along the 180° line of longitude, half a world away from Greenwich.

Why it is needed

Each time zone to the east is one hour later. Go all the way round the world eastward and you would add 24 hours without the date ever changing, which makes no sense. So one line is agreed where the date jumps. Crossing it going west, you skip forward a day. Crossing it going east, you repeat a day.

It is not straight

The line bends to keep island groups and countries on one date. It swings east around Kiribati, which in 1995 moved its eastern islands across the line so the whole country would share the same working week. Samoa moved to the western side at the end of 2011 to match the dates of Australia and New Zealand, its main trading partners. That year Samoa had no 30 December.

The first and last places

The Line Islands of Kiribati, at UTC+14, are the first inhabited places to start each new day. The last are places at UTC−11 such as American Samoa. American Samoa and Samoa are close neighbours, yet their clocks are a full day apart.

What travellers notice

A flight from Tokyo to Los Angeles can land earlier, by the calendar, than it took off. A flight the other way loses a day. Nothing happens to your age: only the label on the day changes.

Three dates at the same moment

Because the offsets in use stretch from UTC−11 to UTC+14, there is one hour in every day when three calendar dates exist together. Take 10:30 UTC on a Tuesday. In London it is Tuesday morning. In American Samoa, eleven hours behind, it is still 11:30 pm on Monday. In the Line Islands of Kiribati, fourteen hours ahead, it is already 12:30 am on Wednesday. The overlap lasts from 10:00 to 11:00 UTC and then Monday is gone everywhere.

See every offset from UTC−11 to UTC+14 on the UTC offsets page.