what does zero hour of his life"mean?
Jacqueline Kennedy, above the reporters, saw only the casket top. Her head bowed, she stood apart. Sergeant Dugger knew that this was the zero hour of his life. Now, now he must reveal some shred of his grief to her.

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This is originally a military term.  Zero hour means the operation will immediately commence.  Essentially, the time for action has arrived, and it is now.