daylight saving timeday·light-sav·ing time
or day·light-sav·ings time
(plural daylight saving times)
- 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. 
The form daylight-saving time, with a hyphen, is best grammatically; 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.
See also usage notes at the former official name, daylight savings time, with an "s" after "saving".