Ancient man probably measured time by daylight and darkness, or by the rising and setting of the sun. He also probably noticed that as the earth turned, the sun was directly over his head only once a day, in the middle of the day. This he called midday.
But this midday time was different in different parts of the ancient world since the sun is seen differently, depending on the location from which man was looking at it.
When travel was slow hundreds of years ago and no radios, telephones, or television connected all parts of the world, this was not too much of a problem. But as people all over the world needed to be more and more in contact with each other, a correct reference to time was needed.
In 1884, an international conference was held in Washington, D.C. to set up a time system for the world. Using knowledge man had gained from the ancient Babylonians, who had divided the circle of the earth into 360°, the scientists divided the earth into 24 time zones, each one covering 15° of longitude, and totaling 360°. Each time zone differed from the one next to it by one hour, since the earth rotates 15° in one hour.
The conference also decided that the starting point of noon would be an imaginary longitudinal line running through Greenwich, England. Heading east, the next time zone, one hour later, would be 1 P.M. when it was noon in Greenwich. Heading west, the next time zone would be one hour earlier, 11 A.M., when it was noon in Greenwich.
Since New York is 5 time zones west of Greenwich, or 5 hours earlier, it is 7 A.M. in New York when it is noon in Greenwich. Tokyo, Japan, is 9 time zones east of Greenwich, or 9 hours later, so it is 9 P.M. in Tokyo when it is noon in Greenwich.
The place on the globe where the point 12 hours east of Greenwich meets the point 12 hours west of Greenwich is called the International Date Line. It is exactly us, halfway around the world, 180° from Greenwich, and runs through the Pacific Ocean.
A traveler heading west from the United States towards Japan on Sunday would cross the International Date Line and find that it was then Monday. When he returned from Japan, say on Tuesday, he would cross the International Date Line and find himself back on Monday.
A traveler crossing the International Date Line can actually live through 2 Tuesdays (or 2 of any day) in one week!