Hi Jeremiah, since the only way to pull in a specific task's planned completion date into a project report is via a collection (see the How to Add a Collection to a View article), you will not be able to sort on the value in the column. I'd encourage you to consider creating a task report instead for this purpose. When you pull "Project-level" fields into task reports, you can make them look a lot like project reports to the end users by carefully labeling columns. --Narayan
In addition to Narayan's suggestion, if the Task level date in question doesn't tend to change very often, another approach would be to create a custom date parameter on the Project itself (e.g. Target Date, or Primary Sorting Date), manually type that date in, and then sort your Project report on that custom data pararmeter.
Crude, but effective; and if worth it, automatable.