"I have not, and cannot, make a promise that we can deliver by that date." You might mean that as "I've been given 6 months. I expect to need 12 months if ever

"I have not, and cannot, make a promise that we can deliver by that date." You might mean that as "I've been given 6 months. I expect to need 12 months if everything goes perfectly and there are no surprises. With surprises, 15-18 months seems realistic." but you haven't communicated that. It's likely that the CEO is hearing "I can't guarantee six months... We'll be cutting it close." and s/he is keeping the bullseye at 6 months and expecting it to take seven. Or maybe s/he thinks that they can rein the scope back in (if necessary) with a month remaining and hit the deadline. Be explicit with your concerns.