International Date Line Explained: What Is Its Role in Universal Timekeeping?

An imaginary line is assigned on the surface of the Earth, which runs from the North Pole to the South Pole in the middle of the Pacific Ocean. When a person crosses this boundary, they may either gain or lose a day, depending on which way they are going. The traveler may seem to move backward or forward in time, but this is actually due to the presence of an international date line (IDL).

Establishment of International Date Line

The international date line was created in 1884 during the International Meridian Conference held in Washington, D.C. The conference was convened by President Chester A. Arthur, who, just like other world leaders, realized the necessity for an international agreement on time and longitude.

The conference resulted in the recognition of the prime meridian in Greenwich, which had already been used for several decades as the world's single "initial meridian." Other prime meridians were in existence at the time, but the Greenwich meridian was adopted since over two-thirds of ships already used it for zero longitude.

A total of seven resolutions were passed at the end of the conference, and the international dateline was established as a result of the Third Resolution. According to this resolution, meridian longitude would be considered in two directions up to 180 degrees.

The resolutions were not binding, and it was up to each county if they would enforce the proposals. It is not a matter of international law, and it is also not implemented by an international governing board. It is just accepted and adopted by all nations since it is crucial for time measurement, instantaneous communication, global interconnectivity, and consistent international databases.

IDL and Time Zones

The first system of time zones was proposed in 1876 by Sir Sandford Fleming, a Scottish engineer who helped in designing the Canadian railway system. He wanted to make the railroad more efficient and prevent any conflicts from the different schedules set by various train stations, which set the time according to their local astronomical conditions.

Fleming proposed a system of 24 standard time zones that span the circumference of the Earth. Within each time zone, all clocks would be set to an average time that best represented the location of the Sun in the sky.

How do time zones and international date line work? For instance, London, England, is eight time zones away from San Francisco, California. Moving eastward from San Francisco to London, a person moves ahead eight time zones, which means skipping forward eight hours. When the time in San Francisco is 1:00 p.m., the time in London is 9:00 p.m. on the same day.

If a person is to move eight time zones in the opposite direction from San Francisco at 1:00 p.m., traveling west across the Pacific Ocean to Yakutsk city in Russia is a bit complicated. It is not simply a matter of subtracting eight hours from the San Francisco time. Instead, it starts by removing an hour for each time zone moving west, making it to 9:00 a.m. At that point, the traveler will hit the international date line, and everything will change. It now requires adding a full 24 hours to the calculations and jumping a full day ahead, making it 9 a.m. the next day.

Another factor must also be considered. Since all the time calculations are taken from Greenwich, four more hours must be subtracted from that time because Yakutsk lies four time zones west of the international date line. This means that when the time in San Francisco is 1:00 p.m. on Wednesday, it is 5:00 a.m. on Thursday in Yakutsk.