term arising from the military use of a 24 hour clock (0800 for 8 AM and 2000 for 8 PM- for those people who can't tell if its day or night) e.g. zero three hundred is 3 AM. zero dark hundred is usually inferred to as being sometime after last call and before reville...anytime between 2 and 5 AM. In other words, Waaay to fucking early to get up and go on watch, or on a 20 mile hike.
Whaddaya mean 'patrol'? it's still zero dark hundred!