Merlin keeps track of the following three kind of values: planned, expected and actual.
You can enter planned and actual values, expected values will be automatically calculated by Merlin. When you have a task and you enter a given planned earliest start in the inspector or the according column, you place your task on a specific start date in the time line.
Move not started activities:
If you move the Gantt chart element of a not yet started activity, Merlin will understand that you are changing the planned values, and adjusts the given planned earliest start date accordingly.