Why am I not allowed to modify the Actual Start and Actual Finish fields?
To edit the Actual Start and Actual FInish fields, go to menu Format – Insert Column – select Actual Start/FInish – click OK. Type in the desired value in the Actual Start or Actual Finish field.
As a workaround, after you created your plan, you can also set a baseline in order to see a difference between Planned Start Date and Actual Start Date. In this way, the Scheduled Start date is in fact the Baseline Date.
After this, if your task actually starts on other date you can change the Start Date for that task. This will be the Actual Date. So, the Actual Date is in fact is the Start Date and the Scheduled Date is the Baseline Date you set in the application when you initially created your plan.