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Zero hour vs Zero time

zero hour

1. (Military) the exact time at which operations are to begin:

  • Zero hour for the bombers to take off was midnight.

2. a crucial moment; critical time:

  • On the day of the championship game, as the zero hour came near, the players grew nervous.

zero time—(also: time zero) the time set as a fixed point of departure in computation:

  • Stoddart measured the extension of the leaf from a zero time defined as the start of the light period 24 days after sowing.

Note: Neither expression correlates in meaning with the phrase zero dayused of information obtained or discovered before it is publicly available:

  • It’s a good idea for [computer] security vendors to collect zero-day information so they can provide preemptive protection capabilities to their products.