什么是国际日期变更线？
The International Date Line, set up in 1884, runs through the middle of the Pacific Ocean and roughly follows the line of 180 degrees longitude. It lies halfway around the world from the prime meridian, the line of 0 degrees longitude that passes through Greenwich in England.
The date line separates two calendar days. When you cross it, you become a kind of time traveler! Cross it heading west, and it's suddenly one day later. Cross back again, and you've "gone back in time".
Despite its name, the International Date Line has no official status under international law, and countries are free to choose which date they use. Although it generally runs from the North Pole to the South Pole, the line zigzags around national borders, such as those in eastern Russia and Alaska's Aleutian Islands. It also bends around several Pacific island nations, which is why places like Tonga and Samoa are among the first in the world to see each new day.
国际日期变更线设立于 1884 年，它穿过太平洋中部，大致沿着经度 180 度的经线分布。它和本初子午线（也就是穿过英国格林尼治的 0 度经线）正好隔着半个地球。
日期变更线把两个日历日分隔开来。越过它的时候，你就成了某种意义上的“时间旅行者”！向西越过日期变更线，日期会突然往后推一天；再越回来，你就“回到了过去”。
虽然名字里有“国际”二字，国际日期变更线其实并没有国际法上的正式地位，各个国家可以自由选择使用哪个日期。这条线大体上从北极一直延伸到南极，但它会绕开一些国家的边界弯弯曲曲地走，比如俄罗斯东部和阿拉斯加的阿留申群岛一带。它还绕过了几个太平洋岛国，所以汤加、萨摩亚这样的地方，是世界上最早迎来新一天的地方之一。