When a task really must start or finish on or near a specific date, you can apply the appropriate fixed-date constraint for that task, such as the Must Start On constraint or the Must Finish On constraint. But applying unnecessary fixed-date constraints can prevent Microsoft Project from calculating a schedule properly. For instance, if you specify that a task must start on a certain date when it doesn’t have to, you might prevent Microsoft Project from scheduling that task to start at the earliest possible date. In general, the earlier your tasks start (and finish), the earlier your project finishes.
Sometimes, project managers apply a fixed-date constraint when they don’t intend to (for instance, if they enter a specific start date and finish date for a task after they enter the task). If you link tasks, you don’t have to enter dates in the Start and Finish fields because Microsoft Project will do it for you.
You can also inadvertently apply fixed-date constraints by dragging the pointer on the Gantt Chart to create a task bar (and thus a task). If you create a task this way, by default Microsoft Project constrains the task depending on which way you drag the pointer. If you drag the pointer from left to right, Microsoft Project will constrain the task to start no earlier than a date. If you drag the pointer from right to left, the task will be constrained to finish no earlier than a date.
You should check all tasks with fixed-date constraints to make sure each constraint is necessary and hasn’t been applied by mistake.