zero hour

Reader's Digest １１月号の記事 "Today's Best Fiction" から引用します。
Hostage Taker
ZERO HOURZERO HOUR, 6:47 A.M. Good day, New York!
It's 41 degrees right now in Midtown, with heavy rain and fog for your morning commute. Luckily, we expect these soggy conditions to be out of here by Lunch time. But bring a fleece lining for those raincoats, because temperatures will continue to plummet throughout the day.
"ZERO HOUR" とは何時の事でしょうか？　辞書を見ます。
・Oxford English Dictionary: The time at which a planned operation, typically a military one, is set to begin.: A few minor adjustments were made to the presentation plan, and we set 3 o'clock as zero hour.
・Collins Dictionary: a critical time, esp at the commencement of an action: Fifteen minutes before zero hour the five guerrillas reassembled.