Continuous Duty Overnight. The FARs state that you have to have at least eight hours of rest before a new duty block can start. Rest being defined as the time from when you block in to when your next check in is. I always get a little nervous when they do that thing where they move back the push-back time by a few minutes in order to accommodate crew rest... it means the crew has had a maximum of 6 hours of sleep, probably less.

So when you have a CDO it is the same 'duty block' as the night before, because there was less than the 8 hours.