The development of what is now known as UTC time began as a collaboration between 41 nations , officially agreed and signed at the International Meridian Conference , in Washington D.C. in 1884 . At this conference , the local mean solar time at the Royal Observatory , Greenwich in England was chosen to define the " universal day " , counted from 0 hours at Greenwich mean midnight . This agreed with the civil Greenwich Mean Time used on the island of Great Britain since 1847 . In contrast astronomical GMT began at mean noon , i.e. astronomical day '' X '' began at noon of civil day '' X '' . The purpose of this was to keep one night 's observations under one date . The civil system was adopted as of 0 hours ( civil ) 1 January 1925 . Nautical GMT began 24 hours before astronomical GMT , at least until 1805 in the Royal Navy , but persisted much later elsewhere because it was mentioned at the 1884 conference . In 1884 , the Greenwich meridian was used for two-thirds of all charts and maps as their Prime Meridian
International Meridian Conference
International Meridian Conference
Time