Summer time / winter time

In France, we put the clocks forward or backward in summer and winter. We call it "passer à l'heure d'été" in the summer, "passer à l'heure d'hiver" in the winter. There is also the expression "changement d'heure" meaning changing the time.
For the winter time, you must put the clocks backward the last Sunday of October.
For the summer time, you must put the clocks forward the last Sunday of March. 
French have been changing the time since 1974. Why? Officially to save energy.