20 minutes from launch to nuclear detonation.  Most USN Ships take longer than that to get underway.  Most USAF take longer than that to take off, considering the Pilots will have to get to their aircraft from home leaving their families behind.  Most of the U.S. Army at Schofield and the U.S.M.C. at Kaneohe Bay will survive if the target was Honolulu (200 feet to 500 feet directly above Honolulu, Air Bursts are more effective than Ground Bursts), the Mountains will stop some of the Effects.  EMP and lingering EMR will destroy Everything electronic (unlike the Science Fiction Movies), even after the EMP, radio communications (HAM, CB, Radio Transceivers) will not work due to Ionosphere disturbance from the lingering EMR.  Hawaii will be without electricity, electronics for Years.  Potassium iodine tablets (pills) and injectable (intravenous)  Prussian blue will be in high demand for the few survivors.  Alpha and Beta Radiation will be everywhere, spread by the Firestorms.