Daylight Saving Time (DST) begins for most of the United States at 2am local time on the first Sunday of April. Time reverts to standard time at 2am local time on the last Sunday of October.
In the European Union, Summer Time begins and ends at 1am Universal Time (or Greenwich Mean Time). It starts the last Sunday in March, and ends the last Sunday in October.
During Daylight Saving Time, clocks are turned forward an hour, effectively moving an hour of daylight from the morning to the evening.
Note there are many oddities: The nations of the world are sovereign powers that can and do change their timekeeping systems as they see fit. For example, some parts of the US and Canada do not observe Daylight Saving Time, such as the state of Arizona (US) and the province Saskatchewan (Canada). Observance can also be erratic. For example, Chile delayed its changeover date for the Pope's visit in 1987 and for a presidential inauguration in 1990.
For complete and up-to-date information about time change dates and time zones, please seeWorldTimeZone.