Sandford Fleming promoted worldwide standard time zones , a prime meridian , and the use of the 24-hour clock as key elements in communicating the accurate time . He referred to the resulting system as Cosmic Time . At the 1884 International Meridian Conference held in Washington , D.C. , the local mean solar time at the Royal Observatory , Greenwich in England was chosen to define the Universal day , counted from 0 hours at mean midnight . This agreed with civil Greenwich Mean Time ( GMT ) , used on the island of Great Britain since 1847 . In contrast , astronomical GMT began at mean noon , 12 hours '' after '' mean midnight of the same date until 1 January 1925 , whereas nautical GMT began at mean noon , 12 hours '' before '' mean midnight of the same date , 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 . In 1928 , the term Universal Time ( UT ) was introduced by the International Astronomical Union to refer to GMT , with the day starting at midnight . Until the 1950s , broadcast time signals were based on UT , and hence on the rotation of the Earth
International Meridian Conference
International Meridian Conference
Coordinated Universal Time