In October 1884 , the International Meridian Conference at Washington DC adopted a proposal which stated that the prime meridian for longitude and timekeeping should be one that passes through the centre of the transit instrument at the Greenwich Observatory in the United Kingdom ( UK ) . The conference therefore established the Greenwich Meridian as the prime meridian and Greenwich Mean Time ( GMT ) as the world 's time standard . The U.S. time-zone system grew from this , in which all zones referred back to GMT on the prime meridian
International Meridian Conference
International Meridian Conference
Time in the United States