Click on the gear icon below the star icon on a project to open the Project Properties panel. If you are inside a project, you can access its properties by clicking on the Settings button at the top right corner of the screen.
Here you can set Start Date, Due Date and Actual Completion Date of your project by clicking on the "+" icon.
Start Date: The date on which the project is expected to begin
Due Date: The date on which the project is expected to be finished
Actual Completion Date: The date the project was actually completed (You can set this date once the project is already complete)