(chiefly USmilitaryslang) Half an hour past 12(am) midnight, or more formal and exact would be 00:30; The practice of adding “dark” and “thirty” (30) can be used sometimes as a slang term in place of an otherwise specific time a time of night, meaning some time when it is dark out between 00:01 - 05:00 hours local time.
2002, Jonathan Shay, Odysseus in America: Combat Trauma and the Trials of Homecoming:
In the very early morning--0-dark-30--the veterans have the Wall entirely to themselves, which is the main reason that we go at that hour.
