The International Date line is a line running along the 180^th meridian of longitude, but with some adjustments to avoid dividing certain island groups that are bisected by it – like the Aleutian Islands of Alaska, Fiji, Tonga, the Kermadoc groups and New Zealand. It is also the geographical point where the Greenwich Mean Time zones of +12hrs and -12hrs meeting about a 24hr time change.

If you leave Greenwich and go East, for each 15 degrees of longitude you traverse, the time zone moves forward one hour. If were to continue around the world, upon returning to Greenwich you would be 24 hours or one full day ahead of actual GMT. If were to do it in reverse (i.e. westward), you would lose an hour for each 15 degrees and end up 24 hours behind GMT.

To fix this issue, you have to subtract a day when you go east over the line and add a day when you go west over line.

 Due to this quirk, ‘today’ technically happened in hour increments each time we moved time zones and will continue to happen until we get back on Sydney time.

Fun fact:
Up until 1995, Kiribati was split in two by the International Date Line. As a result the eastern part of the country was a whole day and two hours behind the western part of the country and the capital. Astute island workers could legitimately extend their weekend by venturing to the eastern part of the country. When the Date Line was moved, Kiribati was reunited in the same time zone.

The change also meant that even thought Hawaii and Kiribati are located on the same longitude, their time zones are a whole day apart.