You might want to use "Source Task.Planned Completion Date". I usually find the fields by creating a report and seeing what I would use in that report space. So by pulling up an issue report and looking through the fields, I don't see a Task section but I do see a Source Task section. So that would be my first guess about what would work with the issue calculated field. By the way, I see your calculated field is set to text... you might want to set it to Date or Date/Time instead.
