International Date line 國際換日線

交义於白日與黑夜之間的交界（網上圖片）

國際日期變更線即日界線，或稱國際換日線，這條子午線由於穿越陸地，而在陸地變更日期既不方便也不可行，故實際使用的國際換日線是一條基本上只經過海洋表面的折線。

為了解決日期紊亂問題，大體以180度經線為日界線；由於照顧行政區域的統一，日界線並不完全沿180°的子午線劃分，而是繞過一些島嶼和海峽：由北往南通過白令海峽和阿留申、薩摩亞、斐濟、東加等群島到達紐西蘭的東邊。

The International Date Line (IDL) is an imaginary line on the surface of the Earth, established by the International Meridian Conference of 1884, that runs from the north to the south pole and demarcates one calendar day from the next. It passes through the middle of the Pacific Ocean, roughly following the 180° longitude but it deviates to pass around some territories and island groups.

The IDL is on the opposite side of the Earth to the Prime Meridian. The Prime Meridian is used to define Universal Time and is the meridian from which all other time zones are calculated. Time zones to the east of the Prime Meridian are in advance of UTC (up toUTC+14); time zones to the west are behind UTC (to UTC-12).

The IDL and the moving point of midnight separate the two calendar days that are current somewhere on Earth. However, during a two-hour period between 10:00 and 11:59 (UTC) each day, three different calendar days are in use. This is because of daylight saving in the UTC+12 zone and the use of additional date-shifted time zones in areas east of the 180th meridian. These additional time zones result in the standard time and date in some communities being 24 or 25 hours different from the standard time and date in others.

A traveler crossing the IDL eastbound subtracts one day, or 24 hours, so that the calendar date to the west of the line is repeated after the following midnight. Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day. The IDL is necessary to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances in the westbound direction.

Source: http://en.wikipedia.org/wiki/International_Date_Line