You can use the Fixed Dates task constraint when you want to be specific about the exact start date and end date of your tasks. For more information about task constraints, see "Task Constraint Overview."
When you select the Fixed Dates (FIXT) task constraint, you must specify the precise Planned Start Date and Planned Completion Date of the task. The predecessor relationship of the task is ignored.
The Duration field of the task is not editable in this case. Duration is calculated as the difference between the Planned Start and Planned Completion Dates of the task. If the Duration Type of the task is Effort Driven, the number of assignees on the task also affect the Duration of the task.