Where does each day begin?

Ask when today began and the honest answer is: it depends where you are standing. There is no single instant when the whole planet ticks over into a new date. Instead the calendar day has a starting edge, a line out in the Pacific Ocean, and from there the new date sweeps westward around the globe, reaching each place in turn as its own local midnight arrives. By the time the last inhabited islands cross into a given date, the first ones have already been living in it for the better part of a day. A day, in other words, is not a moment the world shares; it is a wave that takes a full rotation to pass.

The day has an edge

That edge is the International Date Line, a mostly north–south boundary running roughly down the 180th meridian in the middle of the Pacific. Cross it heading west and you jump forward a calendar day; cross it heading east and you fall back one. The mechanics of that jump, why the line bends around whole nations, and how you can fly for hours and land "a day ahead", are a story in their own right, told in our guide to the International Date Line. What matters here is simpler: because the date changes at that line, the newest calendar day in the world is always the strip of ocean and islands just to its west, and the oldest is the strip just to its east.

Which zone counts as "furthest forward" is measured against UTC, the world's reference clock. A place that is many hours ahead of UTC reaches any given midnight sooner; a place many hours behind reaches it later. The furthest-forward inhabited zones sit around fourteen hours ahead of UTC, and the furthest-behind around twelve hours behind, and those two extremes, as we will see, are what make a single calendar date stretch so improbably long.

Where a new day begins first

The first inhabited land to enter any new date is the Line Islands of Kiribati, Kiritimati, also spelled Christmas Island, and its scattered neighbours. They keep a clock roughly fourteen hours ahead of UTC, the most forward standard time anywhere on Earth, so when it turns midnight there the new date has only just begun and nowhere else has reached it yet. For the next hour or so, that date exists only in one remote corner of the central Pacific.

Close behind, in the order their own midnights arrive, come Samoa, the Chatham Islands off New Zealand and mainland New Zealand itself, followed by the eastern edge of Australia and the rest of the Pacific rim. The exact ordering shifts with the seasons: several of these places put their clocks forward for daylight saving during the southern-hemisphere summer, which lifts them an hour closer to the leading edge, while the Line Islands, which never change their clocks, stay fixed at the front. Kiribati's place at the very front is itself a deliberate choice: the country redrew the date line to the east of its farthest islands so that the whole of Kiribati would share one calendar day rather than be split across the line, which is how the Line Islands ended up at that world-leading offset. It is one of several oddities we cover among the world's strangest time zone offsets.

Where the day ends last

At the other extreme, the last places to leave a date behind sit around twelve hours behind UTC. The very last are Baker Island and Howland Island, two uninhabited United States possessions near the equator; when they finally reach midnight, every other place on the planet has already moved on. Because nobody lives there, the more meaningful "last" is American Samoa, whose clocks run about eleven hours behind UTC. It sits roughly a day behind the leading edge, so late in the global order that when a new date is already well established in the western Pacific, American Samoa still has almost a full day of the old one left to run.

This is also where the widest time gaps on Earth open up: between the inhabited zones nearest each side of the line. Two such places can be so far apart on the clock that they are effectively a whole day out of step, a contest we settle in our guide to the largest time difference on Earth.

The 26-hour new year, and Samoa's leap across the line

Put the two extremes together and the arithmetic is startling. The furthest-forward zone is about fourteen hours ahead of UTC and the furthest-behind about twelve hours behind, so the same calendar date is live somewhere on Earth for roughly twenty-six hours from end to end , two hours longer than a single rotation of the planet. Nowhere is this easier to see than at New Year. The first midnight fireworks go off around the date line, in Kiribati and its neighbours, and the celebrations then roll westward around the world, country by country, for about twenty-six hours, until the very last places, American Samoa and the empty mid-Pacific, finally see in the new year.

The most vivid illustration of how movable this edge really is comes from Samoa. For most of its modern history Samoa sat on the American side of the date line, keeping company with nearby American Samoa. At the end of 2011 it switched sides, jumping across the line to join Australia, New Zealand and its main trading partners to the west, and it did so by simply deleting a day. Friday 30 December 2011 never happened in Samoa; the country went to bed on Thursday and woke on Saturday. The consequence is one of the odder facts of Pacific geography: Samoa and American Samoa are close neighbours in the same corner of the ocean, yet they now keep calendars nearly a full day out of step, one of them among the first places on Earth to greet each new date, the other very nearly the last.

If you ever want to watch the edge move for yourself, our world clock shows the current local time in cities on either side of the line at once, the simplest way to confirm that, right now, somewhere out in the Pacific it is already tomorrow.