Traveling across the International Date Line (IDL) can get weird—seriously weird. Let’s say you’re looking for a change in scenery from the Edenic climes of Auckland and fall smitten with the whale-filled waters of Vancouver. As far as long-haul flights go, this one doesn’t seem too out of the ordinary—if you fly from New Zealand on a Tuesday, you’ll land in Canada the Monday before since you crossed the IDL from the left. But that’s assuming you’ve crossed the line just once. Because of its jigsaw shape, your journey across the Pacific would more so resemble a game of time-travel hopscotch.
All this to say, asking yourself where the International Date Line is, is only natural. And as it turns out, the origin story of this Parasaurolophus-headed imaginary line is more complicated than you’d think.
How the International Date Line came to be
Portuguese navigator Ferdinand Magellan’s surviving crew on the Victoria first documented the time-skipping phenomenon of global travel on Thursday July 9, 1522, whilst stopped at Cabo Verde. On track to become the first expedition to circumnavigate the world, the Magellan sailing came across a strange error in their log books on the home stretch of their nearly three-year journey: despite their fastidious record-keeping, they had somehow fallen back a day compared to the local calendar. As it turns out, while they kept sailing westward, they continually encountered local noon later than they would have from their point of departure, as every 15 degrees of westward travel approximately corresponds with one hour of later local time. As they traveled a full 360 degrees around the world, their record-keeping ultimately fell a full 24-hours behind by the time they returned to Portugal.
Successive travels, observations, and a beloved Jules Verne novel all about the hijinks and perils of hot air balloon flight would later make common knowledge of the fact that circumnavigation results in the gain (if traveling westward) or the loss (if traveling eastward) of a day, and the idea that this difference in time would require its own place on our maps gradually came into fruition.
Mapping out time took further root in October 1884 during the International Meridian Conference held in Washington, DC, which convened 25 different nations to create a global standard for keeping track of time. The existing status quo was to measure time from a destination’s local meridian, its longitudinal position on Earth. At the precise moment the sun passes a destination’s meridian, which derives from the Latin word “meridies” meaning “midday,” that locale experiences its solar noon. But this subjective practice made for a logistical nightmare when it came to coordinating trade routes, communicating using the newfangled telegraph, and scheduling train times. To create this global standard, the conference sought to establish a “universal day,” a 24-hour date scale synchronized to the beginning and end of a day measured from a designated prime meridian, a zero-degree longitudinal line from which all time zones would be defined relative to. As the late 19th century was the height of British sea power and the majority of countries present defaulted to using British maps for navigation anyway, Greenwich, England, was elected the location of the prime meridian, though not everyone left a fan—France did not adopt Greenwich Mean Time until 1911, and on the whole, the global adoption of a Coordinated Universal Time (UTC) would prove slow, taking several more decades to fully realize.