Samoa Deleted December 30, 2011, to Get Closer to Australia
The Pacific nation jumped across the International Date Line to trade with Australia, forcing every calendar to pretend a Friday never happened.
Computerphile · The Problem with Time & Timezones - ComputerphileImagine you built a simple tool. You enter a day and a time, and it tells you how many seconds ago that was. Then you add a little dropdown menu, so someone can compare right now in New York with five days ago in London. All you have to do is add or subtract a few hours from Greenwich, the reference time near London that the world uses as zero.
That is how the trouble starts. The calls start coming in. Australia is nine and a half hours ahead. Nepal is five and three quarters ahead. England changes its clocks on a different week than expected. It gets worse from there.
Then comes the call from Samoa, a small island nation in the Pacific, sitting on the International Date Line. The Samoan caller says, "we skipped today the other year." The country went to sleep on Thursday, December 29, 2011, and woke up on Saturday, December 31. Friday, December 30, never happened there at all.
It was not a mistake. Samoa had been far behind Greenwich, on the same side of the date line as the United States. It jumped to far ahead of Greenwich, on the same side as Australia and New Zealand. The reason was trade, to make trading with Australia easier.
To understand the move, picture the date line as an imaginary fence down the middle of the Pacific where each new calendar day begins. Step across it and the clock time is almost the same, but the date changes. Samoa picked up the whole country and stepped across, losing 24 hours in one stroke.
For the programmer with the seconds counting app, that missing Friday breaks the math. If the code treats December 30 as a normal day, the count of days between December 29 and December 31 comes out a day too long. One approach is to keep a file that records every time a country changes its time zone. Those changes happen fairly often, but they are usually announced ahead of time, so a careful programmer can keep the file updated and build it into the program.
In Samoa, December 30, 2011, simply never happened.
Questions people ask
What exactly is the International Date Line?
It is an imaginary north to south line through the Pacific Ocean, roughly opposite Greenwich, where each new calendar day starts. Cross it westbound and it becomes the next day, cross it eastbound and you repeat a day, even though the time of day barely changes.
How do computers keep track of all these changes?
There is a file that records when any country has changed its time zone. Those changes happen fairly often but are usually announced ahead of time, so a program can stay correct by keeping that file updated and building it into the code.