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What Is the International Date Line (IDL)?
The International Date Line (IDL) is an imaginary line on Earth's surface defining the boundary between one day and the next.
Last updated 26-Sep-2018
The International Date Line is located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich, London, UK, the reference point of time zones. It is also known as the line of demarcation.

The Dateline Is Not Straight
The dateline runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half. On timeanddate.com's Time Zone Map, the IDL is shown as a black line (see image).
翻成白話：國際換日線不是一條直線，中間還有彎來彎去，純粹是因為國界議題，避免特定國家的時區一分為二。

What Happens When You Cross the Dateline?
When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day.
Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours. For example, if you travel the 1061 kilometers (659 miles) across the dateline from Baker Island to Tokelau you have to add 25 hours, or 1 day and 1 hour.
翻成白話：理論上，從西向東跨越國際換日線，要減一天；從東向西跨越國際換日線，要加一天。但實際上，歹誌不是這樣運作的，是依該國選邊站的時區而定。例如，從西向東，從Baker Island（西經176° 29'）到Tokelau（西經172° 31'，由Atafu、Fakaofo、Nukunonu三個珊瑚島組成），你反而要加1天又1小時。參考網址：https://www.timeanddate.com/worldclock/distanceresult.html?p1=3399&p2=4019

Three Dates at the Same Time
Every day between 10:00 and 11:59 UTC, three different dates on the calendar are in use at the same time on Earth. For example, our Time Zone Converter shows:
 At 10:30 UTC on May 2, it is
 23:30 (11.30 pm) on May 1 in American Samoa (UTC−11),
 06:30 (6:30 am) on May 2 in New York (UTC-4), and
 00:30 (0:30 am) on May 3 in Kiritimati (UTC+14).
翻成白話：如果你遵守國際換日線，表示你同意太極生三儀：英國倫敦時間的每天10:00~11:59，地球會同時有三個日子。例如英國倫敦（UTC）是5月2日10:30，美屬薩摩亞（UTC-11）是5月1日23:30，紐約（UTC-4）是5月2日06:30，聖誕島（UTC+14）是5月3日00:30。如果這個時段，這三個地方有三個嬰兒同時出生，傳統的八字命理師就是聽話照做，直接輸入電腦排盤，得到三根不同的日柱來算命。

Changing Sides of the Dateline
The dateline is not defined by international law. Countries are free to choose the date and time zone that they want to observe.
For example, when the Republic of Kiribati gained independence from being a British colony in 1979 some of the islands were on one side of the dateline, and the rest were on the other. They corrected the anomaly in the eastern half of Kiribati by skipping January 1, 1995 and ever since Kiribati has been the first country to enter the New Year.
In 2011, Samoa changed the time zone from UTC-11 to UTC+13 by shifting the dateline to the west（按：誤，應為east）and removing December 30, 2011 from the calendar. They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons.
翻成白話：國際換日線是政治而非科學的產物，鄰近國家可以自由選擇自己想要的時區。例如：
①基里巴斯共和國，在1979年宣布獨立時，東部島嶼是一個時區，西部島嶼是另一個時區，後來在1995年，東部島嶼直接跳過1月1日，從12月31日變成1月2日，將東西方的時區統一為UTC+14。從此以後，基里巴斯共和國就領先全球迎接新年的第一道曙光！
②薩摩亞獨立國，2011年從UTC-11改為UTC+13，主因澳洲和紐西蘭是它最大的貿易夥伴，而且當地有很多僑胞，為了促進貿易，薩摩亞獨立國改變時區以配合澳洲和紐西蘭時間。
③托克勞群島，基於同樣理由，也效法薩摩亞獨立國改變時區以配合紐西蘭時間。

Drawn up in 1884
The 180° meridian was selected as the International Date Line because it mostly runs through the sparsely populated Central Pacific Ocean. It was decided at the International Meridian Conference in 1884 in Washington, D.C. where 26 countries attended.
翻成白話：國際換日線是政治而非科學的產物，誰高興定在哪就可以定在哪。當初選擇把它擺在太平洋中間，純粹是因為那裡人煙稀少。