Keep adding hours as you go east from London and you reach UTC+12; keep going west and you reach UTC−12. Those two meet on the far side of the world, near the 180° meridian. Something has to give there, and it is the date. This builds on Time Around the World.
Key idea
The International Date Line runs roughly along 180° longitude. Cross it going west and you skip ahead one day. Cross it going east and you go back one day. The clock time stays about the same; only the date changes.
Why it works
Travel east around the world and you set your watch ahead one hour in each zone: 24 hours in all, a whole extra day, by the time you get home. Without a date line, your calendar would be a day off. The date line fixes it: going east you set the date back one day when you cross it, and going west you set it forward. It zigzags around island countries so each one keeps a single date.
Worked example: Crossing west
A ship crosses the date line sailing west at noon on Tuesday. What day is it just after it crosses?
- Going west across the date line, skip ahead one day.
- Tuesday becomes Wednesday; the time stays noon.
Answer: Noon on Wednesday.
Worked example: Two neighbors, two dates
Samoa (UTC+13) and American Samoa (UTC−11) are islands less than 200 km apart. It is 10:00 a.m. on Saturday in Samoa. What day and time is it in American Samoa?
- The difference in offsets is 13 − (−11) = 24 hours.
- American Samoa is 24 hours behind.
- Same clock time, one day earlier.
Answer: 10:00 a.m. on Friday.
Samoa switched sides of the date line in December 2011, so its clocks would match those of its trading partners, Australia and New Zealand. To do it, the country skipped Friday, December 30, 2011, going straight from Thursday to Saturday.
Common mistake
Getting the direction backward. Places just west of the date line, such as Kiribati (UTC+14) and Samoa, are the first to start each new day. Places just east of it, such as American Samoa (UTC−11), are among the last. Going west across the line: add a day. Going east: subtract a day. A way to remember: in the west Pacific, it is already "tomorrow."