Q: Why do we need an International Dateline?
Let me first answer a related question: Why couldn’t it be the same time, day of the week, and date everywhere in the world simultaneously? The answer to that question is that it COULD BE, but there would be complications.
Now to see the truth of that last statement, let us start with New York City and say that the time there is 12pm, aka midnight, on Monday, Sept. 25. One minute later, it would become Tuesday, Sept. 26. Fine and good so far. But further west, say in Cleveland, it would not be so much into the nighttime. And very much further west, it would still be light, because the sun, which moves from east to west across the sky, would not have yet passed over, although it has in New York.
And say you lived so far west, that the time we are talking about in our new system (midnight in New York) occurred when the sun was directly overhead. Would you be happy in calling that 12pm, midnight? And one minute later, would you be happy with the day and date changing to Tuesday, Sept. 26?
So, okay, maybe you are convinced that the time of day has to be different in different places, and so you accept the time zones we know and love.
However, you may ask: can’t at least the day and date be the same everywhere? Let’s look at that possibility with again starting from New York City when it is 12pm, aka midnight. One minute later, it would become Tuesday, Sept 26. In one time zone to the west, it would be 11pm, on Monday, Sept. 25. And so forth, until you are way out in the Pacific where is only noon on Sept. 25. But if you go East from New York the day is not Monday, Sept 25, but already 1 am Tuesday, Sept 26. And that time is successively later as you go through time zones. When you get halfway around the world, to the spot discussed above, it is again noon but on Tuesday Sept. 26. So, you see that although we have only accepted the time zones, the difference in day and date, but not in time of day, as we cross a certain north-south line, occurs naturally once we accept them. That certain north-south line is called the International Dateline.
We could now ask about the location of the International Dateline, but that probably is best discussed from a political point of view. It won’t be done here.
The outcome can be said to be that the people who live near a certain north-south line have to accept their date-and-day-of-the week situation being fairly weird, so that everyone living elsewhere have it relatively normal or what one would mostly want.
And so that niceness for most of us is why there has to be an International Dateline, which is the answer to the original question.
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