Time zones cover the Earth’s surface from pole to pole. Because sunlight reaches only half of the planet’s round shape at a time, the need for different time zones arose. Instead of having the same synchronic time for all countries, the surface of the Earth was divided into longitudes of 15 degrees each.

As you know, the Earth takes 24 hours to rotate on its axis and the direct rays of the Sun pass through one degree of longitude every 4 minutes, thus making it midday at that specific point. For time to change on its hourly basis, each time zone need to cover 15 degrees of longitude in width (15 x 24 = 360 degrees).

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Extra Note

To view convert to time in another country visit http://www.worldtimeserver.com/ and for a detailed desciption on Daylight Saving and why it was implemented visit http://deil.lang.uiuc.edu/web.pages/holidays/DST.html.