The International Date Line: Where Today Becomes Tomorrow
Somewhere on Earth the date has to change. That place is the International Date Line — a Pacific zigzag that countries have moved more than once.
If you travel west, you set your watch back an hour each time you cross into a new time zone. Go all the way around the world that way and you will have set it back 24 hours — and yet you arrive home at the same moment as everyone who stayed put. One calendar day has to be accounted for somewhere.
The International Date Line is the place where that accounting happens. Cross it heading west and the date jumps forward by one day. Cross it heading east and the date goes back by one.
Where it is
The line runs from the North Pole to the South Pole through the Pacific Ocean, roughly following the 180° meridian — the line of longitude directly opposite Greenwich. The location is a convenience rather than a necessity: the 180° meridian passes almost entirely through open water, so very few people have to live with a date boundary at the end of their street.
“Roughly” is doing a lot of work in that sentence. The real line bends and zigzags, in places by more than a thousand kilometres.
Nobody owns it
The date line is not defined by any treaty. The 1884 International Meridian Conference fixed Greenwich as the prime meridian, but it did not draw a date line.
What exists instead is a consequence: each country chooses its own time zone, and the date line is simply the boundary between the places that chose to be ahead of UTC and the places that chose to be behind it. When a Pacific nation changes its mind, map-makers redraw the line to match.
Why it zigzags
Following the line from north to south:
- Bering Strait. It bends east to pass between Russia and Alaska, keeping all of Russia on the Asian side. Two small islands sit in the strait less than four kilometres apart: Big Diomede (Russia) and Little Diomede (United States). They are separated by the date line and by 20 or 21 hours of clock time, depending on the season, which is why they are nicknamed Tomorrow Island and Yesterday Island.
- Aleutian Islands. It swings west again so that the whole Alaskan island chain shares a date with the rest of the United States.
- Kiribati. It makes a huge eastward bulge around the island nation of Kiribati, described below.
- Samoa, Tonga, and New Zealand. It stays east of these, keeping them on the same date as Australia.
Countries that moved the line
The Philippines, 1844
For three centuries the Philippines kept the date of the Americas, because Spanish ships reached the islands by sailing west from Mexico. As trade with Asia grew, this became awkward. The authorities removed 31 December 1844 from the calendar, and the islands have shared a date with their Asian neighbours ever since.
Alaska, 1867
When the United States bought Alaska from Russia, the territory changed sides of the date line and switched from the Julian to the Gregorian calendar at the same time. The two changes partly cancelled out, and the result was one of history’s stranger calendar events: a Friday followed by another Friday.
Kiribati, 1995
Kiribati is made up of three island groups spread across nearly 4,000 kilometres of ocean, and the date line used to run between them. Government offices in the east and west shared only four working days a week, because one side’s Friday was the other’s Saturday.
From the start of 1995, Kiribati put the whole country on the Asian side. Its easternmost islands, the Line Islands, moved to UTC+14 — the furthest-ahead time zone in the world. One of them, Kiritimati (Pacific/Kiritimati), is among the first inhabited places to see each new day and each new year.
Samoa and Tokelau, 2011
Samoa had been on the American side since 1892. By 2011 most of its trade and family ties were with Australia and New Zealand, and being almost a full day behind meant losing two shared business days every week.
So Samoa moved across, from UTC−11 to UTC+13 (Pacific/Apia). At midnight at the end of Thursday 29 December 2011, the date became Saturday 31 December. Friday the 30th never happened there. Neighbouring Tokelau made the same change. American Samoa, a short flight away, did not — so the two Samoas are now a day apart.
Three dates at once
Time zones in use today run from UTC−12 to UTC+14, a spread of 26 hours rather than 24. That has a curious consequence: for two hours in every day, between 10:00 and 12:00 UTC, three different calendar dates are in use at the same time.
At 10:30 UTC on a Wednesday, for example:
| Place | Offset | Local time |
|---|---|---|
| Baker Island (uninhabited) | UTC−12 | 22:30 Tuesday |
| London | UTC+0 | 10:30 Wednesday |
| Kiritimati, Kiribati | UTC+14 | 00:30 Thursday |
The lost day of the first circumnavigation
The problem was discovered the hard way. When the survivors of Magellan’s expedition reached the Cape Verde islands in 1522, after sailing west around the world for nearly three years, they found that their carefully kept ship’s log was one day behind the calendar on shore. They had not miscounted. They had seen one fewer sunrise than the people who stayed in Europe.
Jules Verne used the same effect in reverse in Around the World in Eighty Days: Phileas Fogg travels east, gains a day without realising it, and wins his bet.
What it means for travellers
- Westbound across the Pacific (for example Los Angeles to Tokyo), you lose a calendar day. A flight that leaves late on Monday night lands on Wednesday morning.
- Eastbound (Tokyo to Los Angeles), you gain one. It is normal to land earlier on the same calendar day than you took off.
- Hotel bookings, connecting flights, and meeting invitations should always be checked in local time at the destination. The time zone converter shows the weekday for each city so a date change is obvious at a glance.
The International Date Line is a good reminder of how much of our timekeeping is agreement rather than physics. The Sun rises when it rises. Which day we call it is up to us — and, as Samoa and Kiribati have shown, open to revision.