[gadders]

A project plan is only a point in time prediction of a project end date. It should be continuously updated as new facts are known (scope changes, additional complexity, unknown risks happen).

Obviously as you progress through your project, you predicted end date should become more and more accurate.

A project plan, though, is a prediction of the future. I've not seen anyone that can predict the future perfectly. That's not to say that you shouldn't do it, but defective project management creates a project plan on Day 0 and then tries to bend reality to meet the plan.

*Obviously all the above is caveated with reasonableness - you do try and bend reality a certain amount to meet your plan, and you try and keep to your plan as much as possible.