I want to set the end date, and have my project manager set the duration which will in turn change the start date. Is this possible?
Warren,
Yes, just use Start-to-Finish predecessors with a milestone, and plan backwards (it's called "hard deadline" planning, see Help reference here - and search for "hard deadline").
It will look like this:
1. Set a milestone with the deadline, then in the PREVIOUS row put the number of the deadline row as SF predeccessor:
I hope this helps.
Atus