Throughout history , the time is determined in different ways , according to the technological development of the time. The simplest method is to wait for the sun is highest in the sky ( the zenith ) , and at that time will be 12 noon .
To unify the record time, at an international conference in 1884 in Washington ( attended by 25 countries) a " meridian 0" passing through the Greenwich observatory in Britain was determined. From meridian 0, 15 degrees of circumference are added , leaving the Earth divided into 24 meridians that form called Schedule time zones .
So to know when certain place, enough to know what time zone passes closest to the place ( part-time) . If that place is east of the meridian of Greenwich, many time zones are added from the " 0" (Greenwich ) to the time zone where the place is .
If the place is west of Greenwich , time zones are subtracted .
This system has been called GMT , Greenwich Mean Time , and is not very accurate because it is based on the Greenwich Mean Time .
Currently used for accurate purposes , UTC , coordinated universal time, which is obtained from atomic clocks that are in different laboratories in several countries.
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