On January 01, 2009 at At 11.00am GMT, the island nation of Samoa became the last nation to officially enter 2009, 25 hours after the first nation.
Samoa is in the South Pacific, about half way between Hawaii and New Zeeland.
In the South Pacific the International Date Line swings east so that Wallis and Futuna, Fiji, Tonga, and New Zealand's Kermadec Islands have the same date, but Samoa is one day earlier.
The International Date Line (IDL) is an imaginary line on the surface of the Earth opposite the Prime Meridian which offsets the date as one travels east or west across it. Roughly along 180° longitude, with diversions to pass around some territories and island groups. It mostly corresponds to the time zone boundary separating +12 and −12 hours Greenwich Mean Time (UT1). Crossing the IDL traveling east results in a day or 24 hours being subtracted, and crossing west results in a day being added.
For two hours every day, however, at Coordinated Universal Time (UTC) 10:00–11:59, there are actually three different days observed at the same time. At UTC time Thursday 10:15, for example, it is Wednesday 23:15 in Samoa, which is eleven hours behind UTC, and it is Friday 00:15 in Kiritimati (separated from Samoa by the IDL), which is fourteen hours ahead of UTC. For the first hour (UTC 10:00–10:59), this phenomenon affects inhabited territories whereas during the second hour (UTC 11:00–11:59) it only affects an uninhabited maritime time zone twelve hours behind UTC.
The first date-line problem occurred in association with the circumnavigation of the globe by Magellan's expedition (1519–1522). The surviving crew returned to a Spanish stopover sure of the day of the week, as attested by various carefully maintained sailing logs. Nevertheless, those on land insisted the day was different. This phenomenon, now readily understandable, caused great excitement at the time, to the extent that a special delegation was sent to the Pope to explain this temporal oddity to him.
The International Date Line can cause confusion for airline travelers. The most troublesome situation usually occurs with short journeys from west to east. To travel from Tonga to Samoa by air, for example, takes approximately two hours but involves crossing the International Date Line, causing the passenger to arrive the day before they left.
If someone circumnavigates the globe in an airplane from east to west (the same direction as Magellan), they should subtract one hour for every 15° of longitude crossed, losing 24 hours for one circuit of the globe. But 24 hours are added when crossing the International Date Line (from east to west). The International Date Line must therefore be observed in conjunction with earth's time zones: the net adjustment to one's watch is zero.
If someone crosses the International Date Line at precisely midnight, going westward, one skips an entire day; while going eastward, one repeats the entire day.
The effect of ignoring the date line is also seen in Jules Verne's book Around the World in Eighty Days, in which the travelers, led by Phileas Fogg, return to London after a trip around the world, thinking that they have lost the bet that is the central premise of the story. Having travelled the direction opposite that taken by Magellan, they believe the date there to be one day later than it truly is. (info from Wikipedia)