handling a fixed scope with a fixed deadline

I know the iron triangle and how to handle it in 'normal' situations, but I already faced in my career several projects with governments where there is a fixed set of regulations starting with a fix date.

Obviously my approach was to start the project as early as possible to handle it.

But if I do not have any time buffer, what would you use as an approach to do it in any way?

* Talking with the government about the deadline to break out of the iron triangle would be the best option, but there is a risk that they do not agree.