zero hour

American  

noun

1. the time set for the beginning of a military attack or operation.

2. the time set for the beginning of any event or action.

3. a decisive or critical time.

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zero hour British  

noun

1. military the time set for the start of an attack or the initial stage of an operation

2. informal a critical time, esp at the commencement of an action

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Etymology

Origin of zero hour

First recorded in 1915–20

Example Sentences

Agency workers will be included in a ban on "exploitative" zero hour contracts as part of ammendments to the government's employment reform bill, the BBC understands.

From BBC • Mar. 3, 2025

The panel has called his bluff at the zero hour, playing the other side of the same game.

From Los Angeles Times • Oct. 13, 2022

And considering that journalists have long used the platform to document the country’s suppression of press freedom, press outlets view Twitter’s case as a sort of zero hour for Indian free expression rights.

From Slate • Jul. 21, 2022

This is zero hour, and those who don’t do the right thing now won’t get a second chance.

From Seattle Times • Sep. 7, 2021

Modified Julian Date is simply the Julian Date less 2,400,000 days and 12 hours, putting the zero hour at midnight on November 17, 1858.

From "Zero: The Biography of a Dangerous Idea" by Charles Seife