Time zones are the only way to decide on the date and time for the countries of the world. In order to precisely measure and use time, everyone in the world would like to decide on the noon as the time at which the sun is at its highest point in the sky or when it crosses the meridian. This is impossible without knowing the time zones. As the earth rotates at 15 degrees every hour, the sun is at its highest point in the sky at different times in the day for different countries around the globe.
The time zones are decided based on the fact that the world can be split into 24 zones, 15 degrees each. The person who proposed this was Sir Sanford Fleming, a Scottish-born Canadian, in the late 19th century. Although well-accepted by the scientific community all over the world, this system was instituted worldwide only in 1929, the most crucial system of the modern world. Time zones are a solution to complex geographic dependence, but still allowing local time to approximate the mean solar time. http://newtimezone.com
Earlier the time zones were specified based on a point called Greenwich Meridian or Prime Meridian, which is in the Royal Observatory, Greenwich, England. It’s called the Greenwich Mean Time, which was established in 1675. Before the standardization of time zones people used solar time, this had a drawback that each place had a different time based on their geographical location. In order to have a more accurate calculations, in recent years Coordinated Universal Time is followed. Coordinated Universal Time is based on an atomic time scale. If a countries time is UTC+5, it means that when its noon in Greenwich, its 5 pm in that country.
Another important factor for Time zone is the International Date Line. International Date Line is an imaginary line at 0° longitude on the opposite side of the world from the Prime Meridian. International Date Line was never a straight line, it’s been zig zagged to avoid confusion in time for some countries, and even then there is some practical difficulties still lingering round. This will have an effect on the travelers across the globe. If someone crosses this line from west to the east a day is subtracted, if crossed from east to the west a day is added.
Usually the time zones differ by an hour. Even there are countries of the world which have an offset time zone like India, Nepal where the time zone differs from half an hour to fifteen minutes. Russia has the credit of having more number of time zones as it counts to eleven different time zones and china the largest country with a single time zone.