I enter in the date for tasks completed using the Table view. On my screen I have both the table form and the gant shown.
To update the progress of a project, I click on the "View" drop down menu, move curser down to "Table: ____ >" The underscore represents the view you are currently looking at. Me, I often go back and forth between Table: Entry and Table: Tracking. Then move the cursor over to the next drop down menu, and select "Tracking". Once this is done, the table headings will change from "Start/Finish" to " Act. Start/Act. Finish". NOW is when I enter in the completion dates.
My management likes to see the Entry form of the table, so they can see the projected finish dates of the project. I do all of my updating in the Tracking table form.