Ask HN: How to deal with clients that want a signed agreement on a deadline

When dealing with client work what's the best way to deal with a client that want's a signed agreement on a deadline?

My best guess is to explain to the client why software projects miss the deadlines so often, explaining that if we follow this way we cannot have reviews on the project requirements as the project evolves and that I'll have to significantly raise the estimate so I can throw money at any unexpected/unantecipated difficulty.