Hi Margaret,
If your duration is greyed out, this could be because of a customization that prevents you from changing the duration or because of another reason. I would have to have a look at the specific Project your trying to update in order to determine the cause. The start date of tasks can be set upon the initial creation of the project or dependencies in a project template. It can also be caused by the work policy - i.e. if your work policy is Fixed Resource Units, and you update the planned effort, then the duration of the task would be automatically calculated.
For further clarification, we suggest you register for one of our daily Q&A webinars (see the link-> http://www.clarizen.com/resources/online-demo.html#second ).These are great resources for free consulting with our Customer Success Managers and visual answers to your questions. One of our experts will be able to explain it in detail in a much faster manner.
Best regards,
Vanessa