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BIJNDER_MARMATT
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The concept of time is built into the biological rhythm of living things. In this scheme, time and sunlight are inextricably linked: Morning glories open and roosters crow at dawn, night-blooming jasmine unfurls and fire-flies begin their amatory signaling at dusk. Time operates on a larger, seasonal scale as well. Geese and other birds migrate at the same time each year; bears hibernate at the onset of winter.
Early humans responded to the same kinds of biological rhythms, but over time they developed ways to standardize measures of time in order to predict and gain control over events. The positions of the sun and the other stars in the sky during Earth's rotation provide the fundamental unit: the day. The Earth does not rotate at a uniform speed, So the length of all solar days in a year have been averaged, giving us a 24-hour day.
Back when there was little communication among different areas, each town set its clocks by observing the sun's position. When it was noon in Washington, D.C,. it was 12:12 p.m. in New York City. As transportation systems advanced, especially the railroads, the need for a standardized time system increased. In the United States, this happened in 1883, when the country's 60° of longitude was divided into four time zones. All the localities in a zone would observe the time at the center of the zone. These four zones still broadly define today's eastern, central, mountain, and Pacific time zones.
Soon a system of global time zones was created, using the longitudinal line that runs through Greenwich, England, as the prime meridian. Since then, continuing small adjustments have been made to national and international time-zone systems.
WHERE DOES ONE DAY END AND THE NEXT BEGIN?
In theory, the international date line represents the 180° meridian of longitude. When this meridian is crossed from west to east, the date advances by one day. In crossing the line in the opposite direction, from east to west, the date reverts by one day. In reality, however, a number of situations cause adjustments to the international date line. Political frontiers, such as Russia's Kamchatka Peninsula, push out the line to keep a nation's territory intact, datewise. Similarly, the line takes detours to keep together all islands in some Pacific Ocean groups. These local modification date line off the 180° meridian for much of its length.