As the United Kingdom grew into an advanced maritime nation , British mariners kept at least one chronometer on GMT in order to calculate their longitude from the Greenwich meridian , which was by convention considered to have longitude zero degrees ( this convention was internationally adopted in the International Meridian Conference of 1884 ) . Note that the synchronization of the chronometer on GMT did not affect shipboard time itself , 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 , eventually led to GMT being used worldwide as a reference time independent of location . Most time zones were based upon this reference as a number of hours and half-hours " ahead of GMT " or " behind GMT "
International Meridian Conference
International Meridian Conference
Greenwich