Comment by yachtman

Estimates for anything sufficiently complex are pure fantasy. They just serve to place an engineer in a bargaining conundrum and makes them implicitly decrease the scope for the 1000's+ decisions they will need to make during the course of implementation that they are not going to deliver the desired (also implicit) expectations. Its better to work from deadlines, then at least the engineer can try to hedge down the scope and deliver something they are comfortable with.