Hi Alan,
The true answer is: you do not set the start date of a task in Project, Project will calculate it.
However, you can steer Project's calculations in many ways. I think your idea is to use constraints.
These are the possibilities
TheTask.Start= ...
Mind you, this SETS A CONSTRAINT OF START NO EARLIER THAN;, not of Must Start on as you might expect. Thus you could also use
TheTask.Constrainttype=pjSNET
Followed by Thetask.constraintdate=...
If you really want to fix the start date such that relationships or resource problems cannot move it you use
TheTask.Constrainttype=pjMSO
Thetask.Constraintdate=...
Hope this helps,