Daylight Saving Time is a period of time in the summer when the clocks are set one hour forward, so that people can have extra light in the evening. the time during the summer when clocks are one hour ahead of standard time British Summer Time. System for uniformly advancing clocks, especially in summer, so as to extend daylight hours during conventional waking time. In the Northern Hemisphere, clocks are usually set ahead one hour in late March or in April and are set back one hour in late September or in October. In the U.S. and Canada, daylight saving time begins on the first Sunday in April and ends on the last Sunday in October