Are you wanting to do this because the dates are still to be determined? In
a very real sense you should be entering all of the tasks without dates
anyway. That's not to say the task will have blank dates in the plan once
you enter it. Since a task is a physical activity performed by a resource
that extends over a finite and observable period of time, it simply can't
exist in the plan without the dates where the activity is expected to take
place. As the plan evolves the dates may change but that's not the same
thing as not having dates at all. That's why you use scheduling software
such as Project in the first place. When you input the task information -
name, duration estimate, relationship to other tasks, resources required -
it calculates the dates for you where you should be scheduling it and
displays those in the plan. If conditions change later, the expected
schedule dates will also change but for it to be a plan at all, the element
of time as well as the nature of the task must be present. How else will
you advise the resources when they need to engage in the scheduled activity?