World time is measured by the world clock. Now, everybody should know how this clock, which records time world wide, works.
To begin lets know a bit about its history. World time truly came into existence in November 1995. It all began in 1884, when the International Meridian Conference held in Washington declared the Greenwich Meridian as the Prime Meridian for the measure of Longitude. Greenwich is in London and was the site of the Royal Observatory with the same name until 1946.
The Greenwich Meridian was also adopted as the World Time or GMT (Greenwich Mean Time) and the opening point of each time zone in the world.
Since the duration of a day is 24 hours, World Time is divided into 24 time zones, with longitude differences of 15° or one hour. GMT is mean solar time calculated at midday. This system became internationally official on January 1st, 1885.
On January 1st, 1982, the International Telecommunication Union (ITU), identified UTC (Universal Time Coordinated). This was to bring parity in universal time where there were differences in the duration of a day throughout the year, due to the earth’s rotation.
An international agreement declared UTC time as Greenwich Mean Time, though they are calculated differently. GMT is calculated from noon whereas UTC is measured from midnight. Each World Time Zone is set as an integer offset of hours to UTC time, +7 hours or -10 hours to UTC. If time reads 12:00 (Noon/PM), then people having offset +7 hours, have local time 19:00.
If the time displays a '-' (minus) after the time, this means the day before the current UTC time. A '+' after the time means the place has already entered the day following the current UTC time. Such divisions are necessary to manage time in different countries and continents.
Then there are cities that follow Daylight Saving Time (DST) and are denoted with a star (*) after the city name. In such a system, the clock is set to correctly to display DST.
There is another concept called the fixed time. Here you press the relevant key to select a city and a time on the world time table and you get to know the worldwide time right at that moment.