daylight savings

Wiktionary

1. n. the practice in some places of adjusting clocks forward in the Spring and back in the Fall, usually by one hour, so that the adjustment causes the time to coincide with the greatest period of illumination by the sun over the course of the day.

WordNet 3.0

1. n. time during which clocks are set one hour ahead of local standard time; widely adopted during summer to provide extra daylight in the evenings