About six hours passed between the Soviet's response to the first Kassel bomb (two hours after the first detonation) and the Exchange - during this time, Pandora's box was opened on the battlefield and tactical use of CBRN weaponry was undertaken massively by both sides.
With both sides front lines completely devastated and with larger and larger weapons being deployed (including heavy NATO tactical detonations over supply dumps within the GDR, Czecho and the Kola Peninsula), the Soviets took the decision to launch a massive attack on our ability to continue waging war.
We responded. Within four hours it was over.
So the exchange didn't place until the afternoon in the eastern half of the U.S.? IIRC, it was already 2 pm in Great Britain when the Kassel bomb blew up. I would assume the exchange started at around 6 pm GMT..........