Jagged red line down the right is the International Date Line
The International Date Line around 180°
The **International Date Line** (IDL), also known as just the Date Line, is an imaginary line on the surface of the Earth, going from north to south in the Pacific Ocean. The date becomes one day later as one travels across it in western direction, and one day earlier as one travels across it in eastern direction.

The reason for this effect is that the countries on the western side of the International Date Line, (in or outside eastern Asia) have the time zone 10–12 hours more than Greenwich. And the countries on eastern side of it (Alaska/Hawaii and other areas) have the time zone 9–12 hours less than Greenwich. So when travelling across the line, one's watch has to be adjusted 20–24 hours, depending on the time zones. 

For example, New Zealand is twelve hours ahead of Greenwich Mean Time while Hawaii is ten hours behind Greenwich Mean Time. When travelling from New Zealand to Hawaii the clocks must be switched 22 hours backwards, about one day.

Problems
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Originally, the date line ran along the 180° meridian. This is a relatively good choice, because most of the time, there is no land there. There are however some problems.

### Chukchi Peninsula

Looking from the north, one of the first places where the meridian runs over dry land is the Chukchi Peninsula, which is part of Russia. The whole peninsula has been declared part of the UTC+12. This means that all of Russia (and with it, all of Asia) is on the same side of the date line.

### Groups of islands

There are many groups of islands in the Pacific Ocean. These groups belong to states which do not want some islands on one side of the date line, and the rest on the other. They have therefore decided to move the date line, so that all islands are in the same time zone. Examples of this are the Aleutian Islands, which are on both sides of the 180° meridian. They are part of Alaska, and are therefore all in the UTC-10 timezone. The date line has a *bump* to the west, there.

There are a few Islands, east of the 180°, which belong to New Zealand. It was decided that they should have the same date as New Zealand, so the date line runs east of the meridian there.

Kiribati is a state consisting of many small islands, spread over a huge portion of the Pacific; the meridian runs right through the state. Because the state needed the same date over all its territory, the date line was shifted east, the biggest shift to occur, in 1995. The easternmost island of Kiribati was renamed to Millennium Island, because it was the first part of the world which saw the new millennium.

The Philippines had very good trade relations with Mexico. They therefore decided they wanted the same date, date line to the west of the state. When the trade with China grew, this was inconvenient. They changed, after Monday, 30 December 1844, came Wednesday, 1 January 1845.

### Difference in religions

Jews and Muslims and Christians regard particular days of the week as holy. For Muslims this is Friday, Jews (and a few Christian denominations) regard Saturday as the Sabbath, while most Christians observe Sunday as "the Lord's Day" in honour of the 7th day of creation when God was said to have rested. This works well if the traveller stays on the same side of the date line, but there is a problem when travellers move across it. For the traveller it might still be Friday, but the place where he is might say that it is Saturday.

Related pages
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* Meridian
* Time zone

Other websites
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* A History of the International Date Line