[Lizard]

We do bias towards setting start dates and letting end dates figure themselves out. Eventually we want to implement "as late as possible" scheduling, where you can set the end date of the project and work backwards.

However, in the meantime, here's what I think you want to do:
1) Drag the right end of the gantt bar to your desired end date. This will change the duration instead of start date, but we'll fix that in a minute. (Don't mess with the schedule pop-up list: leave it on "As Early As Possible".)
2) Lock the end date (cmd-click on the end in gantt view, or click the lock in the inspector).
3) Adjust either your start date or duration and the other should be calculated for you.