GMT time is the current time measured on the Prime Meridian. The Prime Meridian is represented by a line at 0 degrees of longitude displayed on a map of the world. The Prime Meridian runs through the borough of Greenwich which is in London, England. Hence the name Greenwich Mean Time. GMT is commonly used as a reference for time differences between cities across the globe and is frequently referred to in international business. Other names for GMT include Zulu Time or Universal Coordinated Time (UTC).
As an example of how it is used, Sydney, Australia is GMT +10 hours whilst Greenwich in London is measured as GMT +0, meaning that Sydney is 10 hours ahead of Greenwich. Conversely, New York runs at GMT-4 hours, representing a time difference of 4 hours behind Greenwich. If the time in Greenwich is midnight, it will be 10 0’clock in the morning in Sydney and it will be 8 o’clock in the evening in New York. In other words you simply add or subtract the GMT difference of any city in the world from GMT time to find the current local time. In this same example, you could also calculate the time difference between Sydney and New York. Sydney is 10 hours ahead of Greenwich while New York is four hours behind. Hence the time difference between Sydney and New York is 14 hours.