About 2258 16 April to 0540 17 April. ADC had several lucky breaks as Chidlaw notes. The ADC duty office had been assigned to Alaska, so he had a feel for the problems there. General Garland was on duty that night in the USAF Command Center, so that was another lucky break.
Chidlaw's after action report shows various problems with air defense at the time. The same type of problems were encountered building up the Japan Air Defense Force. Aircraft deviated from flight plans or didn't even file such plans in a timely manner, Communications were poor and went out all the time. The AF had expanded so fast that radar operators were inexperienced. So there were large amounts of unknown tracks (as opposed to UFO which demonstrated un-aircraft like behavior). Just a couple of years later, the experience factor, better procedures, and equipment had brought down the unknown traces by a large number.
Now as far as air defense alterts that is hard to tell how many more there were. Corbin got wind of a big alert in 1957, but had little info on it. Wish his files had survived.
The Alert was over by 0540. So other sightings did not effect this incident. The funny thing to me, which just shows how little we really know is that Chidlaw complained about communicaions with Northeast Command. USAF Intelligence answer was that they were working AFR 200-2 and this should help. Chidlaw was a no nonsense kid of guy. I'll bet he blew up when he read that!