This is why it is more important to have a crisp understanding of what you need to accomplish. From what I've seen, bringing a project in 'on time' is about risk management. The primary leverage point is actively slicing off the unnecessary parts of the time sinks, as they appear. The way to get a jump start on that activity is to get risk estimates along with the time assessment.

Often the primary issue is fitting a goal that is in flux into time period that is not in flux.