We're currently in the process of reviewing the Expert Skills book for the next edition and you're absolutely correct that the existing formula can result in tasks finishing before they begin. An IF function like the one you suggest would prevent this from happening, and we'll revise the lesson to note this.
This change still isn't completely ideal, as the workbook is only set up to track full days. If you did have a situation where tasks were regularly finished in shorter times, you would probably want to set up a workbook that tracked tasks by hour instead of by day, so that multiple tasks could be assigned to the same day.
Excel isn't really designed for complex project management tasks, so you would probably turn to Microsoft Project for a full-featured project management tool instead of trying to create Excel formulas to do the same task.