oh-dark-thirty
Very late at night, or very early in the morning, a time most people consider an ungodly hour (e.g., 3:00 am, 4:00 am).
I was up 'til o-dark-thirty getting my homework done.
I got up at o-dark-thirty this morning.
O-dark-30 is the time of day morning-drive broadcasters have to be at work.,
I had to get up at zero-dark-thirty this morning just to make it through traffic.
See Amish ass-crack of dawn, buttcrack of dawn, and chicken o'clock in the morning and for other early morning references.