There's no way to put a TBD date in a schedule since by definition TBD is
unscheduled <grin>. As Julie suggested for duration, make a "best guess"
estimate and come back to re-evaluate it later. By te way, you should NEVER
be putting tasks dates into Project except under very specific and rare
circumstances - you don't tell it the dates when you want to do the tasks -
it tells YOU the dates you can HAVE given what needs to be done and the
resources you have to do it with.