When you first create a task, the original start and due dates you choose become the BASELINE start and due dates for that task. These baseline dates act as a reference in case start and due dates are then changed, allowing you to see how far before or after the original dates you are.
Baseline dates can be viewed on any task or sub-task details page below the current date details, on the right hand panel of the page. In this example the current dates have been moved before the baseline dates and there is a blue indicator of how far before. If the current dates were after the baseline dates, this would be red, and if they were the same, it would be green.
Baseline dates cannot be changed once they are set, except by Admin users. If you have these permissions and need to change some baseline dates, you can go to the Edit Task form for any task: there is an option to change baseline dates at the bottom of the form (click the More button to reveal).