What is the explanation for the difference in time?

The earth rotates on its polar axis once in every day - or 24 hours. So over a 24 hour period it turns through 360 degrees. This every time you go 15 degrees of longitude (= 360/24) the solar time is 1 hour earlier or later. The solar time has been modified so that most countries, which do not stretch very far from East to West use the same time throughout.