Zero hour

The term Zero hour was used to establish the time officially appointed for the opening of an attack, usually kept secret at headquarters and meanwhile referred to as "Zero," the actual time being finally made known to the troops to be employed only at the latest possible moment before the attack. Minutes before "zero" were designated "Zero ''minus......" and minutes after were "Zero plus......".


 * If zero hour was 6:30am:
 * zero minus 15 would be 6:15am.
 * zero plus 20 would be 6:50am.