There are no things as unscheduled tasks as far as Project is concerned. As
soon as a task is entered, Project calculates when that task is able to
happen based on the project start date, and the durations of the chain of
linked tasks leading up to it. So "TBD" can't happen since Project itself
always does the 'determining' immediately upon task entry and there is no
way to disable it other than by disabling ALL calculations. This is as it
should be, IMHO, as doing precisely that calculation is the reason software
such as MS Project exists in the first place. Project is not intended to
document a project schedule you have determined (or will determine)
elsewhere - its job is to create the schedule for you from base data. You
don't tell it the schedule you want or even need to work, it tells you the
schedule you'll be able to work. You really shouldn't be entering ANY start
dates except for those rare tasks that legitmately need a constraint to
model driving factors external to the project environment.
Manually entering task start dates may appear to be overriding this
behaviour but that's actually an illusion. Forcing a certain entry into the
Start Date field does not set a start date, it sets a Start No Earlier Than
Constraint. That's not the same thing as establishing a start date, it's
setting an earliest start limit but there are many other factors that might
conspire to make the task be scheduled later than that.
