Calendars and dates

The International Date Line: how one planet keeps three dates at once

Somewhere on Earth a new calendar day has to begin. The line where it begins runs down the middle of the Pacific, bends around islands for political reasons, and produces some genuinely strange arithmetic.

Last reviewed October 5, 2026About 3 min to readSite version v2026.10.1

Why a date line has to exist

Offsets increase as you travel east from Greenwich: UTC+1 in Berlin, UTC+8 in Beijing, UTC+12 in Auckland. They decrease as you travel west: UTC-5 in New York, UTC-8 in Los Angeles, UTC-10 in Honolulu. Keep going in either direction and the two sequences must meet. Where they meet, UTC+12 and UTC-12 sit side by side: the same clock reading, but 24 hours apart, which means different calendar dates.

That meeting place is the International Date Line, roughly along the 180-degree meridian, exactly opposite Greenwich. Cross it heading west, from Alaska toward Russia, and you add a day. Cross it heading east, from Fiji toward Samoa, and you repeat one. Magellan's surviving crew discovered the effect in 1522 when they returned to Spain having sailed west around the world and found their careful logbook one day behind the calendar on shore.

Why the line is not straight

The date line has no treaty behind it. Each country chooses which side of it to be on, and the line on the map is simply drawn around those choices.

Russia and Alaska. The line bends east of Russia's Chukotka peninsula and west of Alaska's Aleutian islands so that each country stays on one date. When the United States bought Alaska in 1867 the territory moved from the Russian side to the American side, and its calendar skipped forward.
[Kiribati](/time-in/kiribati). The country straddles the 180-degree meridian, and until 1994 its eastern islands were a day behind its capital. On January 1, 1995 it moved the Line Islands to UTC+14, the largest offset on Earth, so the whole nation shares a date. That pushed the date line about 2,000 kilometres east in a huge bulge.
[Samoa](/time-in/samoa) and Tokelau. Samoa jumped the line in the other direction on December 29, 2011, skipping December 30 entirely, to share a working week with Australia and New Zealand rather than with the United States. It went from UTC-11 to UTC+13 overnight. American Samoa, a few dozen kilometres away, stayed at UTC-11, so the two Samoas are now 24 hours apart.

The two hours with three dates

Because offsets run from UTC-12 to UTC+14, a span of 26 hours, there is a window every day when three calendar dates exist at once. Take 10:30 UTC on a Tuesday. In Kiribati's Line Islands, at UTC+14, it is 00:30 on Wednesday. On Baker and Howland Islands, at UTC-12, it is 22:30 on Monday. Everywhere else it is Tuesday.

The window runs from 10:00 UTC, when Wednesday begins at UTC+14, until 12:00 UTC, when Monday finally ends at UTC-12. Those two islands at UTC-12 are uninhabited, so if you count only places with people, the window is one hour, from 10:00 to 11:00 UTC, with American Samoa and Niue at UTC-11 holding the oldest date.

The live globe makes this visible: open any country panel and each zone shows a Today, Tomorrow or Yesterday marker relative to your own clock. The day-and-night view shows a different line altogether, the one between sunlight and darkness; the date line is a human boundary and does not move with the sun.

What it means for travellers and schedulers

A flight from Los Angeles to Sydney leaves on a Monday evening and lands on Wednesday morning, with Tuesday effectively vanishing from the passenger's calendar. The return trip can land before it departed by the clock: leave Sydney at 10 AM Thursday, land in Los Angeles at 6 AM Thursday. Nothing has gone wrong; the aircraft crossed the line eastbound and the date repeated.

For scheduling, the rule is the same as everywhere else: pin the meeting to one instant in UTC and let each person derive their local date and time. A call at 22:00 UTC on Thursday is Friday morning in Auckland and Thursday afternoon in San Francisco, and both of those are correct. The meeting planner shows the date beside every participant's time so the day boundary is never a surprise.

See it on the site

Time in KiribatiThe country that spans three zones and the largest offset on Earth, UTC+14.
Time in SamoaA day ahead of American Samoa since it crossed the line in 2011.
Live globeOpen a Pacific country and watch the Today and Tomorrow markers change at the line.