Time around the world is calculated using time zones. There are 24 time zones, which each are labeled as a number ahead of or behind the "center" time zone in Greenwich, England. Find the time around the world by taking the time at one location and then adding or subtracting the time zone difference between the two locations.
Earth's rotation causes the sun to be in different positions depending on where in the world it is perceived, and as time is measured based on location of the sun, this means that time is not a constant for every location on Earth. For simplicity's sake, time zones are separated into one-hour intervals rather than being measured at a constant gradient of the sun.
The lines of time zones are not absolute, and different regions of the world sometimes decide which time zones they belong to for consistency so that they do not have small areas within political boundaries that have confusing time tables. It is important to know what official time zone a specific location belongs to when comparing time, because it may not fall exactly within the technical time zone of the location.
Websites such as timeanddate.com facilitate easy research into time zones and time around the world. Timeanddate.com has a combined official and technical time zone map as well as a clock that displays time in all major cities around the world.