Daylight Saving Time (DST), or Summer Time as it is called in many countries, is a way of getting longer summer days by advancing the clocks by one hour. Under a daylight saving regime, the sun the rises and sets one hour later than during normal time. To make DST work, clocks have to be adjusted one hour ahead when DST begins (usually every spring), and adjusted back one hour to standard time when DST ends (every autumn.
Benjamin Franklin suggested daylight saving time in 1784. In 1916, during World War I, several counties in Europe first adopted the method. Today there are many countries observing DST, and many who do not. This means that time differences between countries are not constant.
Today DST it is almost always 1 hour ahead of normal time, but several other variants have been used during the course of history, such as half adjustment (30 minutes), double adjustment (2 hours), and even adjustments of 20, 40 minutes. Double adjustment was used in several countries for some years in the 1940's. Half adjustment was used in New Zealand in the first half of this century. Sometimes DST is used for longer periods than just the summer. From 3 February 1942 to 30 September 1945 most of the USA had DST all year, including the winter.