Greenwich Mean Time ( GMT ) was originally mean time deduced from meridian observations made at the Royal Greenwich Observatory ( RGO ) . The principal meridian of that observatory was chosen in 1884 by the International Meridian Conference to be the Prime Meridian . GMT either by that name or as ' mean time at Greenwich ' used to be an international time standard , but is no longer so ; it was initially renamed in 1928 as Universal Time ( UT ) ( partly as a result of ambiguities arising from the changed practice of starting the astronomical day at midnight instead of at noon , adopted as from 1 January 1925 ) . The more current refined version of UT , UT1 , is still in reality mean time at Greenwich . Greenwich Mean Time is still the legal time in the UK ( in winter , and as adjusted by one hour for summer time ) . But Coordinated Universal Time ( UTC ) ( an atomic-based time scale which is always kept within 0.9 second of UT1 ) is in common actual use in the UK , and the name GMT is often inaccurately used to refer to it . ( See articles Greenwich Mean Time , Universal Time , Coordinated Universal Time and the sources they cite . ) Universal Time ( UT ) is mean solar time at 0 ° longitude ; some implementations are Ephemeris time and its successor time scales described below have all been intended for astronomical use , e.g. in planetary motion calculations , with aims including uniformity , in particular , freedom from irregularities of Earth rotation . Some of these standards are examples of dynamical time scales and/or of coordinate time scales
International Meridian Conference
International Meridian Conference
Time standard