I am in this situation constantly. Bosses very often don't realize when they're making unreasonable demands, and it is sometimes your job to give them a reality check. But you don't just tell them no. Here's my script:

"OK, I'm totally on board with what you want to do, but I wanted to give you a heads-up about some problems I foresee. You want me to do Really Hard Thing and have it to you by Friday, but as embarrassed as I am to admit it, I don't think I'm quite good enough at Really Hard Thing to have it in a condition I'd want to present to you by Friday. I mean, I'll give it my best, but I felt like it would be irresponsible not to tell you so you could make a decision with all the relevant information. If I can make a suggestion, I think it might be better if we do More Reasonable Thing. It seems like the ultimate goal is to do X, Y and Z, and More Reasonable Thing will still do all of that. Alternatively, is there any wiggle room in the schedule so I can make sure you get it in the condition you're expecting?"