Actually there are conventional and global warfare forums that debate world war 1 2 and 3 global war scenarios that debate from the actual ICBM/SLBM launch to detonation timeline varies from 30 minutes for a silo or gantry launched Sarmat/Satan II ICBM and or a sub close to our East or West coast launched ballistic missile in a depressed trajectory from launch to detonation is 5 to 7 minutes.
Our inbound hostile weapons detection system NORAD using SBIRS, OTHBR, PAVE PAWS and CLEAR detection and linking up with Chyenne Mountain, SAC Nebraska, the Pentagon, the president, the vice president, the Pentagon, The National Security Council and the Joint Chiefs of staff is an unclassified approximation. It is believed that the president has approximately 12 to 20 minutes tops to reach Raven Rock or realistically a classified location that nobody knows exist.
Bottom line. In a real world scenario Nobody in he military chain of command will be notifying any civilian agency about anything. I figure us sheep will figure it out when we witness the sun rise twice.