The international date line is an imaginary border that runs through the middle of the Pacific Ocean and marks the boundary between calendar dates, effectively making it the zero-line for the planet’s time zones.

While such a concept had existed for centuries, it wasn’t until 1884 that the line was formally agreed upon at an international conference in Washington, D.C. The international date line is on the opposite side of the planet from the Greenwich — or prime — meridian that marks zero degrees longitude.

Crossing the line from east to west, people would need to set their clocks forward by a day. Crossing from west to east, they would need to set it back by a day.

This type of “time travel” relates purely to the conventions of the 24-hour clock. It wouldn’t involve actual travel backward or forward through time of the kind seen in science fiction movies.

Although the international date line runs from the North and South poles, it isn’t straight. Like time zones, it zig-zags to follow political boundaries. This means the island nation of Kiribati is a day ahead of American Samoa, despite being hundreds of miles farther east.

The international dateline also marks the border between Russia and the United States off Alaska’s west coast. The islands of Big Diomede in Russia and Little Diomede in the United States are separated by 2.4 miles (3.8 kilometers) of water, with the international date line running midway between them.

Follow a team of AP journalists as they travel through the Arctic Circle’s fabled Northwest Passage: New Arctic on AP News 

Photos by David Goldman

Text by Frank Jordans

Video by David Keyton