Is there a reason why you are using the Task calendar versus editing the project calendar? Project Ribbon - change working time.

As a short term fix - apply the task calendar (if it will let you) to the Project summary task. The Project summary task is still using your definition of day (18 hours) but applying that against the working time in the Standard calendar. (8a - 5:00 pm, Monday through Friday)

You would be better applying the changes (6 days per week, 18 hours per day) in the project calendar and removing the task calendars.