To calculate the time zone an area theoretically falls in, the measurement of the area’s longitudinal coordinate is divided by 15.  GMT was the same as Universal Time up until 1972. It is now a simple time zone. In 1847, the British railroads accepted GMT as a standard. It was calibrated using telescopes. The Meridian, after which GMT is named, is used to represent 0 degrees longitude (although it is not truly 0 degrees). Every place on Earth is measured either in degrees east or degrees west of this line.

The fact that time zones around the world are all calculated as an offset or difference from standard time makes it easy to know the time of a place by just knowing the time zone it falls in and the standard time at the current moment. Calculations can also be done using local time once the expected time difference is known. Most time zones are exactly one hour apart and their time is calculated as an offset from Coordinated Universal Time you can see this in our time zone map or by using our time zone converter to check the time difference between one location and another. Heading west of the Prime Meridian / Greenwich Meridian, time is earlier while heading in an easterly direction time is later. The International Date Line adds 3 to the total of time zones around the world. Additionally, several time zones are 30 and 45 minutes apart. Therefore, there are more than the theoretic 24 time zones around the world.