On average, there is only a 4.5% chance that a person will give a birth on their due date. The margin of error is a full month (two weeks before or after the due date). Is that to say the due date is irrelevant? No. It matters to the extent that it can help set priority. 

If you had to pick, what would you optimize for: the budget (people/money), the due date (time), or what's finished (scope)? 

For my wife and I, we chose our daughter's unassisted development and her mother's health as the priority - the scope; therefore, our investment of time and money (budget) was more flexible. However, there was a limit. While it was improbable to assume our daughter would be born after our money and time ran out, they were constraints to manage.

Can you control all three factors all at once? No.
Can you prioritize which matters most and when? Yes.

| Photo by mauro paillex