Rowvill,
There is no way in Planning to have singular dates and I believe that is deliberate by Product in how Planning and Workfront Core are to be utilized in providing distinction between the products. The idea with Planning is teams input items they would like to get approval/review on to become an executable object (project) inside of Core (where you may have singular date such as a project's planned completion date). In Planning they would be providing dates such as to when we think we'll start and when we'd like to ideally end.
In Contrast, the calendar in Core was updated and allows the ability for the singular dates on work that is being executed so teams could adjust workload as part of the execution phase. On a second note, you could input an idea to include it in Planning and we could ask community to go nuts to upvote it 😄