As the United Kingdom grew into an advanced maritime nation , British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian , which was by convention considered to have longitude zero degrees , adopted in the International Meridian Conference of 1884 . Synchronisation of the chronometer on GMT did not affect shipboard time , which was still solar time . But this practice , combined with mariners from other nations drawing from Nevil Maskelyne 's method of lunar distances based on observations at Greenwich , led to GMT being used worldwide as a standard time independent of location . Most time zones were based upon GMT , as an offset of a number of hours ( and possibly half or quarter hours ) " ahead of GMT " or " behind GMT "
International Meridian Conference
International Meridian Conference
Greenwich Mean Time