How is the time difference between two countries calculated?

The earth has 360 imaginary lines called longitudes or meridians running vertically between the poles. Each of these longitudes is called a degree. The 0 degree longitude passing through Greenwich, near London, is considered as standard and the time of all other time zones are calculated accordingly. The time difference between each longitude (each degree) is 4 minutes. So if it is 12 noon at Greenwich (0 degree), it would be 12:04 pm at 1 degree meridian and so on. In India, the standard meridian is 82-and-half degree. So the time difference between Greenwich and India is 82.5 x 4, which is 330 minutes (5 hours 30 minutes).