Completed Tasks in Gantt chart

Hello, in the Gantt view, when I change a status to completed, what dictates if it is shown as a red "overdue".  Is there a "completed" date that i'm missing, or is it governed by the day I change the card from In Progress  to Completed?

Ideally i'd like to be able to put in a custom completed date and see how much overdue the task took compared to the projected time, i.e. the "Due Date".