Noun

daylight saving time

1. An adjustment of the official time during summer to cause events to be scheduled during daylight hours.
   • 2005: answers.firstgov.gov -- Daylight Saving Time is a change in the standard time of each time zone. ... The American law by which we turn our clock forward in the spring and back in the fall is known as the Uniform Time Act of 1966. The law does not require that anyone observe Daylight Saving Time; all the law requires is that if we are going to observe Daylight Saving Time, it must be done uniformly. http://answers.firstgov.gov/cgi-bin/gsa_ict.cfg/php/enduser/std_adp.php?p_faqid=6880

Usage notes

The form daylight-saving time, with a hyphen, is best for grammar; the phrase "daylight-saving" acts as an adjective describing time. The AP Stylebook and the University Style Guide (of The University of Southern Mississippi) both require this hyphenated form.

Daylight Saving Time, without the hyphen, is the official name in the United States, but is not universally (nor widely) used.

See also usage notes at the former official name, daylight savings time, with an "s" after "saving".