How does daylight saving time work?

Under daylight saving time, clocks go forward one hour in spring, so sunrise and sunset happen an hour later by the clock, and back one hour in autumn. In the US, since 2007, it runs from the second Sunday in March to the first Sunday in November, with changes at 2 a.m. local time. In the European Union, clocks change on the last Sunday of March and the last Sunday of October. Germany and Austria-Hungary were the first to adopt it nationwide, on 30 April 1916, to save fuel during World War I. Around a third of countries use it today; most tropical nations, including the United Arab Emirates, do not, because daylight varies little across the year.