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 day—used 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.