It would be useful for the Duration to be dynamic (e.g. linking cell to a cell in another sheet or to contain a formulae) as part of the scheduling calculation to automatically derive the start/end dates for tasks in a Gantt Chart. It is quite common in a construction project for the Duration element of a task to be based on based a combination of volume and productivity (e.g. total widgets to be installed and productivity rate for installing widgets (i.e. x units/day) and for both/either of these parameters to change. In this scenario, I would not be expect to be able to change the start/end date for the task as both would always be calculated.