Can't delete start date so logic does not flow

I have a start date on all of the activities in my schedule.  It correlates to my data date, but I did not deliberately set it.  Now, my logic does not flow based upon predecessors.  Each item starts on the data date.  How do I cure this?  The system will not let me delete the start date without inputting another.  I believe that my initial set up is probably wrong.  But I have not been able to pin down what I should change.