Crossing the International Date Line

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Latest since the release of Jule Vernes "Around the world in eighty days" we know about that special day that you win when traveling eastwards around the world. You count you days by observing the continuous change of day and night while traveling. Finally you come back to the place you started from and if you counted correctly the folks at home will tell you, your journey took you one day less than you counted... How come? Imagine you do your journey in a very fast plane. The plane is so fast that you camo back on the same day...just imagine. Imagine you do your round the world flight in the morning, starting from Europe. While you are sitting in the plane and flying eastwards you observe the sun through a window. Above Europe you see the sun in the east, you say it is morning. Since your plane is so fast, ten minutes later you are above Asia. Here the sun is right above you, you say it is noon (the earth sphere is lit up by the sun with Asia roughly in the middle of the day side). Ten minutes later above the western rim of the Pacific with New Zealand in the far south you see the sun back in the west. Since you fly twenty minutes already it has moved a bit. But still you would say that it is evening. And another ten minutes later it is dark around you, your plane is on the dark backside of the earth, above the Americas. Since the sun is on the other side of the earth now you say it is night. Finally you came back to Europe. Since your fight took you just about forty minutes it is still morning in Europe, more precisely the mornig of the same day that you left Europe. From your experience of the different daytimes during your journey you could claim that one day has passed since you left. This is true for your counted day-night changes. For the absolute time that has passed it is obviously wrong. But it gets a lot less obvious when the one day you "win" by surrounding the earth is distributed over a longer journey.
In our modern times where all parts of the world are communicating day and night and each place has its accurately defined time zone I could keep track of the day I "win" by watching the accumulation of the time shift compared to home. Turkey, one hour, Iran, two and a half hours, Pakistan four hours, and so on. In New Zealand the time shift is already eleven hours.
On the flight to Chile I had to add another eight hours and at the same time subbstract twentyfour hours because I was crossing the International Date Line. What is that? From the picture of the half-lit earth spere and the corresponding times of the day you can conclude that on the backside, in the middle of the night, at midnight, the date changes. You can imagine a line from the North Pole to the South Pole that devides the part of the earth with one date from the part of the earth with the following date. Clearly there has to be a second line of that kind because the earth is round and the two parts of the earth with the different date also border each other on the other side of the earth. This other line coinsides with a different time than midnight. And still the date changes. Isn't it strange that the date should change somewhere sometime during the day?
Now the line of date change at midnight is moving. In principle the other line could also be moving but people agreed upon putting it in a fixed place. Because of its weired nature that the date changes during the day they put it somewhere were few people are living, across the Pacific. And it is called the Intrenational Date Line. Together with the midnight line it devides the earth into the the two parts with different date. Once a day the midnight line crosses the International date line. At that time all earth has got the same date (Nearly, some small parts near the date line still have a different date because the date line is not really straight).
So the day you "win" when traveling around the world is accounted for when you cross the date line. For me that ment to board a plane in New Zealand on 24 of March at around four in the afternoon and arriving after a night in the plane of around ten hours and a time and date change of plus eight and minus twentyfour hours at around ten in the morning at the same day (16 departure time + 10 flight time + 8 time change - 24 date change = 10 arrival time). But I didn't really catch on that when I was gliding alongside the cloudy Andes towards Santiago...

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