Sounds like a title somebody made up years ago, and just randomly tacked it onto this movie, like Cameron Crowe did with "Vanilla Sky," which he had originally tacked onto "Almost Famous."
“What’s with the hokey title?”
It’s an old jocular military expression....at least it was in the Marine Corps. It means in the darkness and stealth of night.
When Mr Montanabound, while in the Navy, had an early flight scheduled, he would tell me he had to leave at Zero Dark Thirty - before dawn. This was 30 years ago, but I’m sure the expression was used much earlier than that.