I can provide two options Option 1: I suppose you have the Task Mode field set to Manually Scheduled. In this mode, Project will not calculate the dates and you are free to enter user-defined start, finish and duration. So, it is fine to have TBD-01/31/2014 in the Start field when you are not sure when the task will begin.
For manually scheduled tasks, if you want to see the recommended start and finish date based on dependencies, calendars and other factors then insert Scheduled Start and Scheduled Finish. Scheduled Start and Scheduled Finish are read-only.
Option 2: Have your task as auto scheduled. Let Project calculate the start and finish based on dependencies, calendars and other factors. If the date is not yet confirmed, you can have custom field like Start1/Flag1 to indicate this date is tentative.