How do to display the total number of days left to complete the project.

Step 1: Create a UDF(User Defined Field for due date at project/task level).
Administration –> System –> User defined fields –> Project/Task.

Step 2: Go to Reports section select project status report and look into settings to select the due date.
Reports –> Project reports –> Project Status –> settings –> Project/task –> Due date.

Step 3: Add a custom column with appropriate formula to show the exact number of days left to complete the project.
Reports –> Project reports –> Project Status –> Settings –> Add custom column and write the below mentioned formula.
daydiff(today(),DueDateProject)
Click Save and Run report this will display the total number of days left to complete the project.

Note:
The above formula will display the days including Weekly Off.