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, by clicking the downward-facing arrow, on the right hand panel of the page.
Here, the task dates have not been moved from their original start and due dates.
Admin users will soon be able to change baseline start and due dates on a task or sub-task details page.