The International Date Line's theoretical position is exactly opposite the prime meridian (0 degrees, Greenwich), running along 180 degrees longitude. Crossing it from west to east subtracts a calendar day; the reverse adds one. Because 180 degrees runs through the central Pacific and avoids dense population, the choice was accepted at the 1884 International Meridian Conference without serious controversy.
The actual line in practice diverges substantially from a straight 180 degrees. No international treaty fixes its precise path, and each country has the sovereign right to choose its own time zone. Result: the date line zigzags according to political and economic decisions taken by individual nations rather than astronomical convenience.
The northernmost bend lies between Russia's Chukotka Peninsula and U.S. Alaska. When Russia sold Alaska to the United States in 1867, the entire Aleutian island chain became American, so the date line bends west around the western tip of the Aleutians. This keeps even the westernmost Aleutian Islands (around 172 degrees east) on the same date as the rest of the U.S.
An interesting consequence is the Bering Strait between Russia's Big Diomede Island and the U.S. Little Diomede Island. The two are only 3.8 km apart but sit on different sides of the date line, with calendar dates differing by a full day (more precisely, 21 hours). From Little Diomede, you can literally see "the island of tomorrow" with the naked eye.
The largest bend in the date line is around the Republic of Kiribati in the central Pacific. Kiribati consists of 33 atolls scattered across about 5,000 km (60 degrees of longitude). Before 1995, the date line passed through the middle of the country, leaving its eastern islands (Phoenix and Line Islands) on a different calendar date than its western islands (Gilbert).
In 1995, the Kiribati government moved the entire country to the western side of the date line. The Line Islands now use UTC+14, the largest positive offset on Earth, and the date line bulges far to the east to accommodate. As a side effect, Caroline Island (now Millennium Island) became the first place on the planet to greet each new day, drawing global attention during the 2000 millennium celebrations.
On December 29, 2011, Samoa moved from the eastern side of the date line (UTC-11) to the western side (UTC+13). The day after December 29 was December 31, and December 30 simply did not exist for Samoa that year. The motivation was the time difference with Australia and New Zealand, the country's main trading partners.
Before the switch, Samoa was 21-23 hours behind Sydney and Auckland. Samoa's Friday was Australia's Saturday, meaning that of Samoa's Monday-through-Friday business week, only four days (Monday through Thursday) overlapped with Australian business hours. After the switch, the time difference dropped to 3-4 hours, and the business weeks aligned perfectly. Geography pointed one way; commerce won.
No international treaty defines where the date line goes. Each country sovereignly chooses its time zone, which is what places it on one side of the line or the other. The result is a "customary line" on maps with no legal force. If a country changes its decision, the line moves, no negotiation required.
This flexibility means the date line will continue to evolve. As Pacific island nations realign with new economic partners, time zones may shift again, and the line's shape with them. The International Date Line is not a feature of natural science but of politics and economics, perpetually subject to revision by human choice rather than fixed by physical law.
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